For more information, please see full course syllabus of Statistics

For more information, please see full course syllabus of Statistics

### t Distributions

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

- Intro
- Roadmap
- When to Use z vs. t?
- What is z and t?
- Why not z? (Why t?)
- But Don't Worry!
- Rules of t Distributions
- Degrees of Freedom (df)
- t Family of Distributions
- How to Find It?
- Example 1: Which Distribution Do You Use? Z or t?
- Example 2: Friends on Facebook
- Example 3: t Distributions
- Example 4: t Distributions , confidence interval, and mean

- Intro 0:00
- Roadmap 0:04
- Roadmap
- When to Use z vs. t? 1:07
- When to Use z vs. t?
- What is z and t? 3:02
- z-score and t-score: Commonality
- z-score and t-score: Formulas
- z-score and t-score: Difference
- Why not z? (Why t?) 7:24
- Why not z? (Why t?)
- But Don't Worry! 15:13
- Gossett and t-distributions
- Rules of t Distributions 17:05
- t-distributions are More Normal as n Gets Bigger
- t-distributions are a Family of Distributions
- Degrees of Freedom (df) 20:02
- Degrees of Freedom (df)
- t Family of Distributions 24:07
- t Family of Distributions : df = 2 , 4, and 60
- df = 60
- df = 2
- How to Find It? 31:01
- 'Student's t-distribution' or 't-distribution'
- Excel Example
- Example 1: Which Distribution Do You Use? Z or t? 45:26
- Example 2: Friends on Facebook 47:41
- Example 3: t Distributions 52:15
- Example 4: t Distributions , confidence interval, and mean 55:59

### General Statistics Online Course

### Transcription: t Distributions

*Hi and welcome to www.educator.com.*0000

*Today we are going to talk about t-distribution.*0001

*Previously, we learn that there are different situations where we use z and when you use t.*0004

*Today we are going to talk about when to use z versus t.*0011

*We are going to break down and sort of reflect and recognize what is z and t?*0015

*What do they have in common and with is different about them?*0022

*For certain cases we are going to ask question, why not z why t instead?*0024

*What does not z have?*0031

*What is deficient about z? *0033

*We will talk about rules of t distribution, they follow certain patterns and t distributions *0035

*are a family of distributions separated by degrees of freedom. *0044

*Different t distributions have different degrees of freedom.*0049

*We are going to talk about what are degrees of freedom?*0053

*We are going to talk about how degrees of freedom relates to that family of t distribution, and then finally summarize how to find t.*0056

*First off, when do we use z versus t?*0065

*We covered in the previous sections where we look at whether we knew the population parameters or not.*0072

*In hypothesis testing, we frequently do not know the mu of the population, but sometimes we are given sigma for some reason or another. *0080

*In this case we use z in order to figure out how many standard errors away from the mean we are in our SDOM.*0091

*But in other situations, we do not know what sigma is.*0102

*In that case we use t in order to figure out how many standard errors away our x bar is from our mu.*0107

*Just to draw that picture for you remember we are interested in the SDOM because the SDOM tends to be normal given certain conditions.*0118

*Although mu sub x bar = mu given the CLT what we often want to know is if we have x bar that fall here or x bar that falls here.*0126

*We want to know how far away it is from the mu sub x bar.*0147

*In order to find that we would not just use the raw score and get the raw distance but we would want that distance in terms of standard deviation.*0153

*But because this is the SDOM, we call it the standard error.*0165

*We would either want a z or t and these numbers tell us how many standard errors away we are from this point right in the mu.*0168

*What is the z and t?*0181

*The commonality as we saw before is it tells us number of standard error away from mu sub x bar and that is common to both.*0186

*That is what the z score and t score both have in common.*0208

*Because of that their formula looked very much the same. *0213

*For instance, one way we can write the z formula is like this. *0217

*We have x bar - mu or mu sub x bar they are the same and this gives us the distance in terms of just the raw values.*0231

*Just how many whatever inches away, points away, whatever it is.*0251

*Whatever your raw score means, degrees away divided by standard error.*0258

*If we double-click on that standard error and look at what is inside than the standard error also written as sigma sub x bar *0264

*because it is the standard deviation of a whole bunch of mean = sigma ÷ √n.*0275

*If we look at the t score formula then we have almost the same formula.*0284

*We have that distance ÷ how big your little steps are, how big your standard deviations are.*0294

*But when we double-click on the standard error like something on the desktop, you double-click it and open it up what is inside?*0302

*Well, you could also write this one as s sub x bar and that would be s ÷ √n.*0311

*Here in lies this difference right there.*0323

*That is our difference.*0325

*Here the difference is that standard error found using the sigma, the true population standard deviation.*0327

*Obviously if you use the real deal that is better or more accurate than the standard error found using estimate population standard deviation.*0345

*That is s.*0371

*S is estimated from the sample and if we double clicked on s it would look like this.*0375

*It is that basic idea of all the squared deviations away from x bar, away from the mean of the sample.*0383

*X sub i - x bar ^{2}.*0395

*We have all the squared deviations and we add them up ÷ n -1 because this is our estimate of the population standard deviations *0402

*and all of that under the square root sign in order to just leave us a standard deviation rather than variance.*0414

*This is an estimate of population standard deviation. *0421

*It is not the real deal, so it is not as accurate. *0426

*One thing you should know is that the z score is less variable and the t score is going to be more variable.*0430

*That is going to come in to bear on why we use which one. *0438

*Okay, so why not z?*0443

*When we have situations where we do not have the population standard deviation, why not z?*0448

*Why cannot just be like you are using s, why cannot we do that?*0458

*Why do we use t? *0466

*It is because we use s this is actually something a little bit weird.*0468

*The weirdness comes from the fact that this s is much more variable than sigma.*0474

*Sometimes when we get our estimate, our estimate is scat on.*0481

*Sometimes when we get our estimate it is off.*0485

*That is what we mean when it is more variable.*0489

*It is not going to hit the nail and head everything single time.*0491

*It is going to vary in its accuracy. *0495

*Now z scores are normally distributed when SDOM is normal.*0497

*Here is what this means.*0502

*The way you can think about it is like this, when the SDOM is normal and we pick a bunch of points out *0503

*and find the standard error from those points and plot those, we will get another normal distribution.*0516

*But that is not necessarily the case for s.*0523

*Here we need to know that z scores are nice because z scores is going to perfectly cut off that normal distribution accurately for you. *0530

*Remember, the normal distribution it always has that probability underneath the pro and it has these little marks.*0547

*These can be set in terms of z scores.*0557

*What is nice about the SDOM when it is normal is that when we have the z score it will perfectly match to the proportion of the curve that it covers.*0563

*This will always match.*0579

*The problem is t scores do not match up in this way.*0581

*We can just say why do we just call a t score a z score and still use the same areas underneath the curve?*0587

*We cannot do that because that is just the superficial change.*0600

*Here is what we mean by the z scores are normally distributed. *0603

*When you get z scores and when we talk about normal distribution, I'm not just talking about that bell shaped curve.*0611

*Yes overall it should have that bell shaped general shape but it is a little more specific than that.*0619

*You can have the bell shaped and have the perfect normal distribution.*0628

*For instance, 1 standard deviation away this is area will give you 34% of the area underneath the curve.*0635

*This area is about 14% and this area is about 2%.*0645

*That is a true normal distribution. *0653

*This on the other hand, it looks on the surface as if it is normally distributed. *0656

*It looks like that bell shaped curve, but it is not. *0662

*Here is why.*0665

*This area, I should have actually drawn it a little bit differently, but I want to show you that do not go by appearances.*0666

*Appearances can be deceiving. *0677

*This might actually be a little bit less than 34%.*0678

*It might be something like 25%.*0685

*If that was the case, you would see this area and that area is not 34%.*0688

*It is 25%.*0700

*Not only that but this area is now a little bit more than 13 ½, it is around 14%.*0701

*Now this area is not 2% but 11%.*0710

*Although it looks like a bell shaped curve, it is not quite a normal distribution because *0715

*it does not follow that empirical rule that we have talked about before.*0722

*What is nice about z scores is that z scores will always fall in this pattern. *0726

*These z scores will always correspond to these numbers.*0731

*That is why you could always use that z table in the back and rely on it.*0735

*The t scores are not going to do that for you.*0739

*T scores may not give you that perfect 34, 13 ½ and 2% sort of distribution. *0746

*Even though the SDOM might be normal, the t scores are not necessarily normal.*0753

*We had this normal thing and we have t scores and how do we go from t score's defining this area underneath the curve.*0762

*That is the problem we have here.*0772

*It turns out that if n is big then this does not matter as much. *0774

*It n is really large, if your sample size is large then the t distribution approximates normal. *0782

*It goes towards normal but when n is small, then you have to worry.*0788

*Also when n is in the middle or when n is big, it is just large.*0795

*There are all these situations where you have to worry about the t as well as the area underneath the curve.*0801

*If the t scores are not normally distributed then we cannot calculate the area underneath the curve.*0810

*If we have our lovely SDOM and we know that the SDOM is nice and normal and we have our mu sub x bar here then everything is fine and dandy.*0816

*We have x bar here and we want to find that distance, and we find the t score.*0832

*The problem is we cannot translate from this directly into this area.*0838

*That is the problem we ran into.*0844

*Here what we see is this sort of more like a t distribution than a z distribution.*0847

*I'm just going to call the z distributions to call them basically, the normal distribution.*0865

*The t distribution is often a little bit smooched.*0871

*Think of having that perfect normal bell shape.*0876

*It is squishing the top of it down.*0880

*It makes that shape ball out a little bit.*0882

*It is not as sharply peaked but a little bit more variable.*0888

*We had said the s is more variable than the sigma.*0895

*It makes sense that the t comes from s is more variable than the z that comes from sigma.*0902

*You might be thinking what are we stuck?*0911

*We are not stuck and here is why.*0921

*He actually worked out all the t distributions as well. *0924

*He manually calculated a lot of the t distributions and made tables of the t distributions that we still use today. *0928

*He published those tables and under the pseudonym the student.*0944

*At the time he was working for Guinness brewery and he could not publish because they were sort of like we do not want you to know who we are.*0949

*Our secrets are very dark beer.*0957

*He published under the pseudonym and because of that some of your t distributions *0960

*in the back of your book maybe labeled the students t to talk about Bosset’s t.*0967

*Here is what Bosset found, he found that t distribution can be reliable to.*0973

*You can know about them it is just that you need more information when you need for the z distribution.*0980

*For z distribution you do not need to know anything.*0988

*You just need to know z and it will give you the probability.*0990

*Life is simple.*0993

*T distributions are not that simple, but not that complicated either.*0995

*They had a few more conditions to satisfy and the biggest condition that you will have to know is about degrees of freedom.*1002

*Because for each degree of freedom there is a slightly different t distribution that goes along with it.*1012

*Let us talk about some of the rules that govern t distributions.*1024

*The first one you already know as t distribution gets more normal as n gets bigger.*1031

*This makes sense if we step back and think about it for a second.*1039

*Imagine if n=n then what would your s be?*1042

*If your sample is like a entire population then s should be much closer to the actual *1054

*population standard deviation much better than when n is small.*1071

*It is still a little off because of the n-1 thing but it is very close and that is the closest you can get.*1077

*T distributions are more normalized and gets bigger because s is a better estimate of sigma as n gets bigger.*1085

*That makes sense.*1111

*The problem all stems from s.*1113

*It is variability that as s gets better, less variable and more accurate to the population then t gets better.*1116

*T is based on s.*1128

*That is like t distributions are more normalized as n gets bigger.*1130

*T distributions are a family of distribution.*1135

*It is not just one distribution.*1138

*It is a whole bunch of them that are alike in some way and it depends on n.*1140

*It depends technically on degrees of freedom, but you can say it depend on n sometimes because degrees of freedom is often n -1.*1145

*There are other kinds of degrees of freedom this is the one you need to know for now. *1154

*But later on we will distinguish between different kinds of degrees of freedom.*1159

*Degrees of freedom is actually important as a general idea it is just the number of data points -1.*1163

*We have a family of distributions. *1174

*They all look sort of a like.*1178

*They are all symmetrical and they are unimodal and they have that bell like shape, but they're not quite normal. *1179

*Not all of them.*1190

*As n gets bigger, or as degrees of freedom gets bigger the distribution becomes more and more normal.*1191

*Let us step back and talk a little bit about degrees of freedom first.*1201

*Let us assume there are three subjects in one samples so n=3.*1207

*We know that just by the blind formula n -1 degrees of freedom is 2 but what does this mean?*1213

*Here is the thing.*1224

*Let us assume there are three subjects in one sample and let us say it is some score on a statistics test.*1228

*They can score from 0 to 100 and if I say pick any 3 scores you want and that could be those subject scores.*1235

*Your degrees of freedom would be 3.*1244

*You are free to choose any 3 scores.*1246

*You are not limited.*1249

*You are not restricted in any way.*1250

*If you figure out any sample statistic, let us say the mean or variance.*1253

*If you figure out any sample statistic then if you randomly picked 2 of those scores you can no longer just pick the 3rd score freely.*1261

*You have to pick a particular score because you already used up some of your populations for the mean.*1274

*The mean will constrain which two scores you could pick.*1283

*This logic will become more important later. *1288

*Let us put some numbers in here.*1292

*Let us talk about the case when n= 3 and degrees of freedom = 3.*1294

*It would be like there are three subjects and they could score from 0 to 100.*1299

*I am totally free.*1310

*I can pick 87, 52, my last score I can pick anything I want.*1314

*I can pick 52 again, 100, or 0.*1321

*It does not matter.*1325

*I can just pick any score I want.*1326

*If I erase these other scores I will just put in a different score.*1328

*It does not matter. *1333

*I'm very free to vary.*1335

*But let us talk about the most situations that we have in statistics where we figure out summary statistics. *1337

*Here we have n=3 and degrees of freedom =2.*1345

*Here is why.*1350

*The score is the same, it can go from 0 to 100.*1351

*We also found the x bar =50.*1358

*If we found that the x bar = 50, then we cannot just take any score all 3 times.*1363

*Can we pick any score for the first one?*1374

*Yes I can pick 0.*1377

*Can I pick any score for the 2nd one?*1379

*Sure, I can pick 100.*1383

*Now that third score I cannot take any score.*1386

*If I pick 72 my mean would not be 50.*1392

*If I pick 42 my mean would not be 50.*1394

*If I pick another 0, my mean would not be 50.*1399

*That is the problem and because of that if this is my data set so far I have been free to vary.*1403

*I freely chose this guy but this last one I am locked in.*1410

*I have to choose 50.*1415

*That is the only way I can get a mean of 50.*1417

*That is what we call degrees of freedom.*1420

*This logic is going to become more important later on, but for now what you can think about is *1423

*because we are deriving other summary statistics from our sample we are not completely free to vary.*1429

*We locked ourselves down. *1437

*We pinned ourselves down and built little gates for us at the borders.*1439

*Now you know degrees of freedom and we know as degrees of freedom or n goes up we see more and more normal like distributions.*1445

*I have drawn three distributions here for you.*1460

*Here you might notice that I have used basically the same picture of a curve for all three of these.*1462

*You might think they have all the same distribution.*1469

*Not true because you have to take a look at the way that I have shown you that t down here.*1473

*The way that I have labeled this x axis or t axis in this case is really to change our interpretation of these curves.*1482

*Remember what the normal distribution says.*1493

*The normal distribution says 1 standard deviation to the right or positive side, 1 standard deviation *1496

*to the negative side that area should be about 68% of your entire curve.*1502

*Is it true here?*1507

*No it is not, this does not look like more than 50% of the curve.*1510

*This looks like maybe 1/3.*1521

*Maybe a little less than 1/3.*1526

*This is starting to look more like 60% of the curve, but still maybe not quite 68% of the curve.*1528

*It is still only looks like may be 50% of the curve or a little more.*1539

*Imagine that this was shifted in the middle this would be more like 68% of the curves.*1544

*Something like this would be more like 60% of the curve.*1560

*That is how you can see that as your degrees of freedom increases it becomes more and more normal.*1567

*Even this is not quite normal. *1582

*This is not quite 68% but a little bit less actually.*1585

*As the DF gets bigger and bigger that area starts to look more and more like the normal distribution.*1588

*Now there is another way I can draw these pictures and I believe in this other way you can see more easily how helped this is more variable version.*1598

*Remember I am saying that t distribution is like you are stomping down on the peak of it and smooching it out a little bit.*1615

*I believe that if I draw the same picture in a slightly different way you will see why. *1624

*In this case, here is what I have done. *1630

*I have kept the t axis the same and now it is labeled in the same way, but I have drawn these distributions in a slightly different way. *1634

*Now this one is a little wider and this one is less wide and this one is even less wide.*1647

*It becomes more narrow, more like the normal distribution.*1656

*Notice that if I drew the line here, a little bit after 1 standard deviation away we see they are a little of that curve on the side.*1661

*You know if that is 50% and maybe 15%, 10%, something like that. *1675

*This might look more roughly equivalent to this, maybe a little bit less.*1685

*Maybe like 20%.*1693

*This looks like much more than this.*1695

*Maybe this is like 25 or 30% even compared to this.*1700

*In that way you can see using the same concepts the drawing and picture in a slightly different way that this distribution is much more variable. *1706

*It is spread is very wide. *1719

*Where is this distribution is much less variable?*1721

*Remember t is all because of the variability found in s.*1725

*When s is very, very variable and n is very small, s is very variable, so the t distribution is also quite variable.*1731

*As s n gets bigger, s gets more and more accurate, more like the actual standard deviation of the population.*1741

*And because of that, it becomes more and more normal.*1752

*Let us break this one down.*1755

*In degrees of freedom of 60, here is what it might look like.*1761

*It might look something that is very close to our 34, 13 ½ , 2% normal distribution.*1769

*If we drew our little lines there, that would probably look very close to this picture.*1777

*It looks pretty close.*1792

*When we draw something like this, this area might only be 25% of this whole curve.*1797

*This other areas also combined 25%.*1810

*If I split this like this, then this would be something like 14%. *1817

*A little bit less than this but still quite a bit.*1826

*This one might even be more than 14%, maybe like 18%. *1832

*As you can see that in this distribution even though I have drawn it like this and just labeled it differently. *1840

*In reality, it will look more like this if you kept this t axis to be constant.*1849

*It will look sort of smooched out.*1855

*How do you find t at the end of the day?*1859

*How do you find the t and not only that how do you find the probability associated with that t?*1867

*For instance, where t is greater than 2?*1874

*How do you find these probabilities?*1878

*We know how to do it for z but how do you do it for t?*1881

*One thing that you could do is you can look at the back of your book usually in the appendix section *1884

*there is something called the t distribution or the students t distributions that you can look at.*1892

*Oftentimes it will have degrees of freedom on one side like 2, 3, 4, 5 all the way down and then it will show you either one tailed or two tailed area.*1898

*It might give you .25, .10 and .05, .025.*1914

*It might give you these areas.*1926

*The number right here tells you the t score at that place.*1929

*If you wanted to know where the 25% cut off is, what this t score is for degrees of freedom = 2 distribution and you would look right here.*1935

*If you wanted to know it for .025 then you would look here.*1962

*You want to look for degrees of freedom, as well as how much of the curve you're trying to cover.*1975

*That is definitely one way to do it.*1984

*The other way you could do it is by using Excel and just like how Excel will help you find probabilities *1988

*and z scores for the standardized normal distribution you can also find it in Excel for the t distribution.*1995

*It needs a couple of hints.*2003

*Let us start off with tdist.*2006

*Tdist is the case where you want to find the probability but you have everything else.*2012

*What the tdist will do is if you put in the degrees of freedom and you put in the actual x value.*2019

*You can think of the x value as the t value and it will only take positive t values.*2033

*For instance, a t value of 1 and the number of tails if you want this entire area or you just want that area alone.*2039

*You can either put in one or two then it will give you the probability of this area.*2058

*I can show you right here.*2066

*Let us put in tdist for t(1) and degrees of freedom 2 and let us look at what it might say for two tails.*2070

*It will say 42% and if you look at this exact same thing, but if you look at it for one tail it will just divide this area in half.*2098

*21% and 42% makes sense.*2111

*Basically this is giving you this area + this area if you want 2 tails.*2116

*But if you only want one tail it will just give you this area.*2122

*We know that for 95% competence interval we often use z score of 1.96 and that will give us a tail of .025 or if we count two tails 25%. *2125

*Let us see what this gives for 1.96 when we have a degrees of freedom of only 2.*2149

*Let us put in 1.96.*2158

*If we put that in our z score, if we put in 2 tails we would only get 5%, but let us see what we get here.*2163

*Degrees of freedom 2 and number of tails let us put in 2.*2173

*Do you think this should be more or less than 5%?*2179

*Let us think about this.*2183

*The t distribution is like slightly smooched, it is more spread out and because of that it is going to have this longer tail.*2186

*It is not going to be nice and all compact in the middle.*2196

*It will be spread out.*2200

*We would imagine that it have a fat tail.*2202

*I would say more than 5%.*2204

*We see that it is almost 20% a t of 1.96.*2207

*Let us put that same z score in.*2216

*Normsdist this is whenever we want the probability and put a 1.96.*2218

*Here we get the negative side, so we want 1 - and this gives us just 1 tails.*2227

*I am going to change this to 1 tail, so we could look at it. *2241

*Here on one of our tails, one side of it, it is almost 9 1/2% is still out there.*2245

*But when we use the z score only 2 1/2% are still out there.*2253

*Let us look at the same t distribution for a very high degrees of freedom. *2258

*Let us try 60.*2271

*Even with something like 60 we are starting to get very close to the z distribution, but still this guy is more variable than the z distribution. *2273

*Let us see if we could go even higher. *2287

*Instead of 60 I am going to put in 120.*2289

*Notice we are getting closer but still these are more variable than these.*2294

*Let us go a little less.*2303

*Let us go like 1000 and see what happens there.*2304

*We are getting close but still slightly more variable.*2309

*That is a good principle for us to know.*2316

*The t distribution although it approximates normal, it approximates it from one side.*2318

*Here is the normal distribution standards .02499.*2324

*There it is and it is getting closer and closer to it, but it is approaching it from the high-end.*2329

*These numbers are dropping and getting really close to that, but not quite hitting it.*2336

*Now you know how to get the probabilities but what if you have the probably and you want to find the t score?*2345

*What would you do?*2355

*In this case, you would use the inverse t in for inverse.*2356

*Here you would put in the two tailed probability.*2362

*Let us say we want to know what is the t boundary for if we wanted only 5% in our tails?*2366

*Here is the situation I am talking about for this one.*2374

*We had this distribution and we know we want these to be .025, just like a z distribution.*2378

*We want it to .025 but we want to know what these numbers are here.*2393

*We want to know what these numbers are.*2398

*It depends on your degrees of freedom.*2403

*Let us try degrees of freedom of 2, 60, 120, and 1000.*2405

*Let me label this.*2413

*Here we get the probabilities from t dist and here are the probabilities from standardized normal distribution, or the z distribution.*2424

*We do not want the probabilities we actually want the t boundaries themselves and the z boundaries themselves. *2443

*If we want the z boundary at .025 or at 5%, we would use normsin and we put in our probability.*2460

*I forget if it is one tailed or two tailed.*2472

*Let us try one tailed but we would need two tails.*2474

*We get very close to -1.96.*2477

*We just have to memorize that but that is why this is saying at -1.96 you have about 2 1/2% in that little tail. *2489

*Now what about the t?*2501

*In Excel it is inconsistent because z it gives it to you on the negative side, for the t it only gets 2 for the positive side.*2504

*That is confusing but I often do not memorize that.*2512

*I just try out a couple of things until it spits out the thing I'm looking for.*2515

*You have to understand how these things work so that you could predict what's going on. *2520

*We will use t inverse and we want to know the probability and I believe this is going to be two-tailed.*2527

*.05 and degrees of freedom of 2.*2538

*We get .05 and degrees of freedom just to test whether this is one tailed or two tailed.*2543

*Let me put that in.*2563

*I believe you have to give it two tails.*2565

*You have to put in the two tails probability here so that is .05 and the degrees of freedom 2 and this will give us these boundaries.*2570

*This will only give us the positive boundary, but because it is symmetrical, you automatically know the other side.*2580

*This would give us a boundary of 4.3.*2589

*Remember for the z score this boundary will be 1.96 but for a t distribution with the degree of freedom of 2, this would be 4.3.*2593

*That is quite high because remember it is really spread out.*2604

*You got to go way out far in order to get just that 2%.*2609

*What about this boundary for degrees of freedom of 60?*2612

*What do we get then?*2621

*We get something very close to 1.96 but it is a little bigger than 1.96.*2624

*Remember because the t-distribution is more variable you to go farther out there in order to capture just that small amount of .025%.*2630

*That mean 2.5% or .025.*2641

*If we go to 120 we should expect is that boundary to come closer and closer to 1.96 from the big side, but not quite hit 1.96 or more closely 1.9599.*2646

*We are getting close to that 1.96 number, but still it is a little bit higher.*2671

*Finally we will go buck wild and put in degrees of freedom of 1000 we get something very close to 1.96 but still little the higher than 1.96.*2678

*Those are two different ways that you can find the t, as well as the probability that t is associated with.*2692

*Remember the degrees of freedom and you have to know whether you want two tailed probability or one tailed probability.*2701

*As well as your degrees of freedom. *2714

*That is what you will have to know in order to look things up on a t distribution.*2717

*Let us go on to some examples.*2722

*In each of these situations which distribution do you use, the z or the t?*2729

*Is there a 500 million people on Facebook how many people have fewer friends than Diana, who has 490 friends?*2734

*Assume that the number of friends on Facebook is normally distributed and here they give you the sigma.*2742

*We know that you can use the z distribution here.*2749

*Here the researchers want to compare a given sample of Facebook users average number of friends 25 to the entire population. *2753

*What proportion of sample means will be equal or greater than the mean of this group?*2763

*N = 25, but the mean is 580.*2772

*They have an average of 580 friends.*2779

*Here I definitely would not necessarily use z but I also do not have the standard deviation.*2783

*Maybe this is connected to the previous problem.*2796

*If so, if I assume that they come from the whole population and they give us the information for the whole population here.*2800

*If sigma = 100 then I will use z.*2811

*This one I probably left out some information.*2816

*What about this last one? *2820

*Researchers want to know the 95% competence interval for tagged photos given that a sample of 32 people *2822

*have an average of 185 tagged photos and a standard deviation of 112.*2829

*Here it is very clear, since I know s but I do not know the sigma for tagged photos.*2835

*I only know the sigma for friends, but not for tagged photos.*2844

*In this case, what I would do is use the t distribution because I will probably have to estimate *2848

*the population standard deviation from the sample standard deviation.*2855

*Example 2, what we get is that problem and we just have to solve it. *2860

*There are 500 million people on Facebook but how many people have fewer friends than Diana?*2869

*Here it is good to know that we do not need a sampling distribution of the mean.*2874

*We do not need the SDOM.*2880

*In fact, we are just using the population and Diana.*2882

*We could draw the population and it tells us that the population is normally distributed.*2886

*Number of friends is normally distributed and so the mu = 600 and a standard deviation is 100.*2895

*This little space is 100 so this would be 700.*2914

*Diana has 490 friends so here would be 500.*2920

*It is asking how many people have fewer friends than Diana?*2929

*How many have that?*2937

*It is tricky because this will give us the proportion but it would not give us how many people?*2940

*What we will have to do it multiply that proportion to the 500 million.*2950

*This is all 500,000,000 and that is 100%.*2956

*We will need to know some proportion of them that have friends fewer than Diana, fewer than 490.*2961

*We will have to figure that out and so we will have to multiply 500 million by the percentage. *2974

*Let us get cracking.*2981

*We can figure out the z score for Diana and that would be 490 - 600 and ÷ 100.*2984

*I only need to do standard error if I was using the SDOM but I am using the population standard deviation.*3010

*That is often helpful to draw this.*3016

*Here we have about 100 ÷ 100 = -1.1.*3018

*The z score of -1.1 and I want to know the proportion of people who have friends less than Diana.*3031

*You can look this up on the back of your book, so I would just look up the z score of -1.1 or you could put it into Excel normsdist -1.1.*3046

*I should get about .1357 so that would be .1357.*3065

*That is about 13 ½ % of the population have fewer friends than Diana.*3081

*What I want to do is only get 13% of these entire populations and that would be 500 million × .1357. *3089

*You can do this on a calculator, so that × 500 million = 67.83 million.*3103

*Do not forget to put the million part.*3117

*It is not that you only have 67 people who have fewer friends than Diana.*3124

*That would be our answer right there.*3129

*The researchers want to compare a given sample of Facebook users average number of friends a sample of 25 to the whole population.*3132

*What proportion of sample means will be equal or greater than the mean of this group?*3146

*Here I'm going to assume because there is no other way to this problem.*3159

*I am going to assume that we could use the information from example 2 because we are talking about the same thing, the number of friends.*3165

*We actually know the population.*3173

*The population is approximately normally distributed with the mu of 600 and standard deviation of 100.*3176

*Mu= 600, standard deviation=100 and from this I need to generate an SDOM because *3195

*now we are talking about samples of people not just one person at a time. *3205

*Because of that I need to generate SDOM for n = 25.*3211

*The nice thing is we already know the mu sub x bar = mu that is 600 but we actually also know *3216

*the standard error because standard error is standard deviation ÷√n.*3234

*In this case, it is 100 ÷ √25 =20.*3240

*1 standard error away here is 20. *3246

*This would be 580, 560, and so forth.*3255

*It is asking what proportion of sample means will be equal to or greater than the mean of this group?*3262

*Equal to or greater than means all of these and they are just asking for proportions we do not have to do anything to it once we get the answer. *3271

*Well, it might be nice if we could actually get the z score for this SDOM.*3281

*Here, instead of just putting 580 I would want to find the z score here.*3290

*Here are friends but I want to know it in terms of z score. *3296

*It is actually really easy because it is the z score of -1 and we can actually just use the empirical rule to find this out because we know at the mean, *3303

*at the expected value we know that this is 50% and this is 34%. *3318

*If we add that together, the proportion of sample means greater than or equal to the mean *3327

*of this group that = the proportion where z score is greater than or equal to -1 and that is .84%.*3341

*Final example, researchers want to know the 95% competence interval for tagged photos given that *3357

*a sample of 32 people have an average of 185 tagged photos and a standard deviation of 112.*3366

*Interpret what the CI means.*3375

*Here we do not know anything about the population, but we do know x bar which is 185 *3377

*and we do know the standard deviation of the sample s which is 112.*3386

*We also know n is 32.*3393

*Remember when we talk about competence interval we want to go from the sample to figure out where the population mean be.*3396

*What we do is we assume that we are going to pretend SDOM here and we assume that the *3408

*x bar is going to equal the expected value of this SDOM which is 185.*3420

*From there we could actually estimate the standard error by using s.*3428

*Here mu sub x bar = 185 this is assumed not sigma but x sub x bar is s ÷ √n=112 ÷ √32.*3438

*If you pull up a calculator you could just calculate that out 112 ÷ √32 and get 19.8.*3460

*We know how far the jumps are and because we used s we cannot just find the z score we have to find t score.*3477

*We will have to use t score in order to create a 95% competence interval.*3496

*Although I do not know what the t distribution for the degrees of freedom of 32 – 1.*3504

*I do not know degrees of freedom of 31 t distributions looks like.*3515

*We will have to figure that out.*3520

*What we eventually want is this to be .025.*3524

*These are together a combined two tailed probability of 5% and we will have to use t inverse because we already know the probability.*3531

*We want to go backwards to find the t. *3544

*T inverse and we put in our two-tailed probability .05 and put in our degrees of freedom, which in this case is 31.*3548

*We ask what is the t and it says it is 2.04.*3559

*The t right here at these borders is 2.04 and because it is symmetrical we also know that this one is -2.04.*3564

*In order to find the competence interval we are really looking for these raw values right here.*3577

*In order to get that we add the middle point and add 2.04 standard errors to get out here and we subtract out 2.04 standard errors to get out here.*3587

*The competence interval will be x bar + or - the t score. *3605

*How many jumps multiplied by how big these jumps actually are and that is the score right here multiplied by s(x).*3615

*If we actually put in our numbers that is going to be 185 + or -2.04 × 19.8.*3627

*If you just pull out a calculator we could get 185. *3638

*Make sure to put that = 185 even I forget sometimes +2.04 × 19.8 and remember Excel knows order of operations. *3643

*It will do the multiplication part before it does the addition part,*3660

*The upper limit will be 225.39 and the lower limit will be 144.61.*3664

*I just rounded to the nearest tenth and this would be 225.4 and this would be 144.6. *3683

*We need to interpret what the CI means. *3697

*This means that there is a 95% chance that the population mean will fall in between 144.6 and 225.4 that is the interval.*3705

*That is it for t-distributions. *3721

*Thank you for using www.educator.com.*3724

0 answers

Post by Terry Kim on October 20, 2015

In example 1, last part isn't standard deviation = sigma?

and variance = sigma squared. Since standard deviation = sigma is given shouldn't we use z? not t?

2 answers

Last reply by: Terry Kim

Tue Oct 20, 2015 12:22 AM

Post by Kedrick Mckissock on March 14, 2015

How do you know when a question is talking about the real SD and not the estimated population SD

0 answers

Post by Brijesh Bolar on August 20, 2012

degrees of freedom well explained..