For more information, please see full course syllabus of Statistics

For more information, please see full course syllabus of Statistics

## Normal Distribution: PDF vs. CDF

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.

## Mathematics: General Statistics

## Transcription: Normal Distribution: PDF vs. CDF

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

*Today we are going to be talking about normal distributions again but this time breaking it down into the PDF *0002

*or probability density function and CDF or the cumulative distribution function.*0008

*Here is what we have for today.*0014

*We are going to be talking about frequency charts which we have been doing before *0018

*and then contrast it to this new idea but still fairly elementary, cumulative frequency of charts.*0022

*We are going to do a very brief review of calculus.*0031

*It is going to be a very elementary review, no actual calculations just conceptual.*0034

*Then we are going to talk more deeply about the probability density function and the cumulative distribution function.*0042

*You have to reprogram yourself to see PDF and not think of it as a portable document or format.*0048

*Let us talk about frequency versus cumulative frequency.*0056

*So far what we have been doing is we have some sort of variables such as score on SAT verbal.*0061

*We have all these values that value can potentially hold.*0067

*We have been talking about what percentage of our sample or population have achieved that score.*0072

*1% score is 800, 3 score is 750.*0081

*That is what we have been talking about so far.*0086

*Here when I write percentage I am talking about relative frequency but it is largely the same thing as frequency. *0089

*Not a big deal.*0100

*When we talk about cumulative percentile, what we are really talking about is not just the people *0102

*who has achieved that value but an accumulation of everybody who has come before it.*0112

*Let us start off at the bottom.*0118

*Here only 1% of the population has achieved 250 points.*0120

*It think that is one of the minimum or something.*0126

*But 3% have achieved 300 or below.*0129

*If you are in the third percentile, you have out performed 3% of every else that has taken the test.*0136

*Not something to write home about yet.*0144

*This 8% actually accounts for this 3% as well plus a little bit extra.*0147

*The 16% encapsulates everybody who has come before it.*0163

*Cumulative percentile is helpful is you want to know your ranking in a performance.*0169

*For instance you want to know what percentile of the population you are in.*0175

*You just do not want to know what percent of everybody who is around you has also achieved that score.*0179

*You want to know how many people you have out performed.*0185

*Cumulative percentile continuously adding all the people that have come before you.*0188

*It gives you that ranking.*0195

*If you are in the 95 percentile, you know out performed or equally 95% of that sample.*0198

*That is cumulative frequency is really helpful to us.*0210

*One of the things that you want to notice is that cumulative frequency, when you just look at the number right away, *0215

*you do not know how common that score is.*0222

*When you look at 98% you do not know how common 750 as a score is but you could easily find that out *0226

*just by looking at the difference between 98 and whatever cumulative frequency came before it.*0235

*That difference is 3, so 3% of people have been in that bracket.*0240

*Another thing about cumulative frequency I want you to notice is that it is a monotonic increase.*0247

*It means that there is no going up and then going back down.*0254

*There are no changes in direction.*0260

*It is continuously going up, up, and up.*0262

*That makes sense because you have to add up everybody who has come before you.*0265

*That is cumulative frequency.*0269

*When you look at it on a visualization, you could see what I mean by monotonic increase.*0272

*Here we have an example of monotonic increase.*0280

*This curve goes up, and up, and up because it is adding up everybody who has come before you.*0286

*You have to be at least the score before you or higher.*0293

*Every score is improving on the previous score.*0298

*Whereas in frequency we can have non monotonic curves because here you could go up, down, we could have the uniform distribution.*0304

*We could have all kinds of things but in cumulative frequency distribution you cannot have a uniform distribution.*0315

*If everybody has only see the bottom score then you would have a uniform distribution because you will be adding 0 everytime.*0321

*Otherwise the most frequent shape that you will see is a monotonic increase that looks like this.*0329

*Here we see just like this normal looking distribution and what ends up happening when you have this normal-ish distribution *0336

*you have the s shaped curve when you transform it into the cumulative frequency.*0345

*This part in the middle, that part corresponds from about 400 to 600.*0352

*That part corresponds to the biggest jumps and as you see there are big jumps here too but the jumps are just going in one direction.*0362

*That is how it looks.*0377

*Let us put that on hold for a second and let us talk about calculus in brief.*0382

*A lot of people when they think about calculus, they think immediately derivatives and intervals like integrating stuff and getting derivative of equations.*0397

*It is what they think about.*0398

*Let us start packing and think about it conceptually.*0399

*Calculus in some way you could think of an equation as being on a continuum.*0403

*Let us just say that there are some equation that we are thinking of Y=x ^{2} or something like that.*0409

*We all know what that function looks like.*0418

*It looks something like this a parabola and when we think about this we are saying this is just plotting exactly what y is given from x.*0421

*That is all that graph is.*0434

*When x is 0 y is 0.*0435

*When x is 1 y is 1.*0437

*When x is 2 y is 4.*0439

*We are just plotting those precise points.*0442

*When you go and take the integral, I’m going to put the integral on this side versus you go and take the derivative, *0447

*you are describing 2 different aspects of the same graph.*0459

*When you talk about the integral, what you are doing is you are no longer plotting these particular point but now you are plotting these areas.*0467

*When you think about integral, think about area.*0480

*Whenever you have some curve or shape or line, when you take the integral you are pointing towards the area.*0485

*You could get these areas of this weird bizarre shapes and at the same if you go on the other side of the continuum *0495

*and you go towards the derivative which you will end up getting is not the area but instead you are just getting the slope.*0504

*Here instead of being interested in the points themselves, now you are interested in these little slopes.*0516

*All these little slopes, that every single, tiny point.*0526

*You are really plotting those slopes.*0531

*You are interested in these changes.*0536

*Here I want you to think slope and obviously for any equation so even when you get a graph of slopes, you could get the slopes of slopes.*0540

*Even when you have the slopes of slopes, you could get a slope of slopes of slopes.*0552

*You could go more and more towards that derivative side or on the other hand if you get a graph of whole bunch of areas you could get the areas of that curve. *0556

*You could go more and more towards the integral side as well.*0566

*This is going to be important to us because we are going to be interested particularly in a curve that looks like this, the normal curve PDF.*0575

*What we want to do is get the cumulative areas of that one.*0585

*Which direction should we go?*0592

*We should probably go towards the integral direction because what we want is the cumulative areas of this function.*0595

*That is going to be an important concept.*0605

*Now let us talk about the PDF or what we call the probability density function.*0610

*We have talk about how the standard normal distribution is a little bit different than just the normal distribution.*0617

*The mean is 0 and the stdev is always one because of that it is a special case that is very helpful to us.*0622

*There is a special sign we use just for the PDF of the standard normal distribution.*0638

*That is what we call little yi and it looks like this.*0644

*When it is written out sometimes it looks like a lower case y.*0648

*That is how people sometimes write it.*0654

*That is what it looks like.*0657

*That is the symbol we use to denote that we are going to be talking about standard normal distribution.*0659

*The function actually looks something like this.*0667

*X just represents some value on your variable of interest and there are different ways you could write this but the heart of this is e ^{-1}/2x^{2}.*0674

*That is the heart of this and even this ½ is like a constant, I just remember it as e^-x ^{2}.*0683

*That is the heart of the shape, all divided by 2pi.*0694

*Obviously this can be written in slightly different ways you might also see it as 1/√2pi multiplied by the exponential function to the power -1/2 x ^{2} or –x^{2}/2.*0715

*There are couple of different ways you could write that.*0735

*There function might seem a little bit crazy to us but let us break it down.*0738

*You might want to use www.wolframalpha.com or if you have a graphing calculator feel free to use that.*0743

*That will work largely in the same way.*0750

*If you go to www.google.com, www.wolframalpha.com is like a combination of fancy graphing calculators / Wikipedia for math and science stuff.*0754

*It is helpful.*0767

*Here we could write any function we want.*0771

*Let us start off with just y= e^ -.5 x ^{2}.*0778

*Let us start with that part first and let us see what we got.*0802

*What www.wolframalpha.com will do is that it will actually draw the equation for us.*0807

*Notice that we have something that looks like a normal distribution but one of the issues is we just want to divide it by the √2×pi.*0811

*That will just change the shape of it very slightly.*0834

*Notice that largely it is the same basic fundamental shape.*0840

*You divide by that constant to give you a couple of properties of a normal distribution that are going to be important to us later.*0844

*The heart of this function is that exponential function and it is in particular exponential function to the x ^{2} power.*0851

*That is what gives us that nice curve.*0866

*That is what we think of little yi, that is this equation.*0871

*The PDF for a normal distribution that we know has any kind of mean and any kind of standard deviation.*0877

*One thing that I forgot to point out is that when you look at this, one thing that you will notice is that then mean or point of symmetry is 0 and 1 stdev out.*0888

*If you go 1 stdev out that looks about like 68% of that curve.*0913

*It seems like more than half.*0923

*This depicts the mean being 0 and stdev being 1.*0926

*If we want a regular normal distribution that did not have a mean of 0, it did not have stdev of 1, what would the formula for that be?*0934

*What would the function for that be?*0948

*The general equation for the PDF, we do not have a special symbol, we just use the regular symbol that we use for all function f(x).*0951

*Here it is still this function at the heart of it but all we are doing is we are going to be adding in mean *0966

*and stdev as variables so that you could put in whatever mean and stdev that you want.*0975

*Let us start with the heart part.*0981

*It is e^ -, instead of ½ we need to change that x ^{2} and it is going to be x – mu because we are going to put in that mu.*0985

*If mu is 50 we want that point of symmetry to be over 50.*1005

*If mu is -5 we want that point of symmetry to be over -5.*1010

*We want to square that.*1015

*We are squaring that distance.*1018

*Here we have done that actually except that is x – 0 ^{2}.*1020

*That is what we could just convince it is x ^{2}/ 2Ʃ^{2}.*1030

*That is where that ½ comes from and when Ʃ is 1 it is just -1, right?*1038

*That is why we do not see that crazy part in here.*1046

*There is just one more thing, 2 pi Ʃ ^{2}.*1051

*Another way you could think about it is having 1/Ʃ × phi, instead of just putting an x we are putting in x – mu / Ʃ.*1059

*If you out this function in here and you substitute x with all of these crap and you multiply by 1/ Ʃ then you would get this equation.*1080

*This is the simplified version.*1093

*Now let us put this in www.wolframalpha.com and let us see what kind of function we have.*1096

*We could put in a mu(550) and Ʃ(100).*1102

*Let us see what that normal distribution looks like.*1114

*I will put this up here.*1120

*We want to substitute in 550 right here and 100 right here.*1127

*Let us put in e^ - ( x – mu(550)) ^{2} ÷ 2 × Ʃ^{2}.*1135

*I’m just going to make that 100 ^{2}, that is 10,000.*1160

*I’m just going to make sure that it is all in this parentheses here.*1166

*All divided by √2×pi × Ʃ ^{2}.*1179

*I’m just going to put my 10,000 again.*1191

*What we should see from this is the mean being at 550 and the standard deviation being at 100.*1195

*Let us see and the nice thing about www.wolframalpha.com is that just in case I missed a parentheses *1204

*it will rewrite the input for you in a more standard form instead of this linear way.*1220

*So that you could check and see that you are missing a parentheses or something.*1229

*Is this is where 550 is and that looks like the center of that curve and not only that from 550 if you go out to 450 or 650, *1234

*that does looks like about 68% of that curve.*1249

*What we see here is that this function, if you substitute in any mean and any stdev it will effectively draw or represent this normal distribution for you.*1253

*That is the PDF, but what this gives you is at every point what is the probability for that particular value of x. *1269

*Let us move on to cumulative distribution function.*1281

*We want the cumulative distribution function.*1287

*We do not just want that curve.*1292

*Instead what we want is a cumulative adding up of all the areas that came before.*1298

*What we are looking for is that curve and at any point I can tell you the percentage we are at.*1306

*Here, it encapsulates all the space that came before it.*1314

*As we talked about before, because I want that cumulative area, all we have to do now is take that integral of phi.*1323

*We represent the cumulative density function as upper case phi rather than lower case phi and we put in x.*1331

*All we do is we take the integral and we take the integral from –infinity up to x, whatever x is and that will give you the area so far of phi.*1348

*Obviously you could actually get the integral but I’m just to leave it as it is because I just want you to know what the function actually means.*1367

*The meaning is just the integral of little phi which we talked about.*1380

*What that gives you is this idea of the area up to this point of x, whatever x is.*1387

*Because we are talking about for a standard normal distribution that is why I’m using that phi or else I would use f(x) for the normal distribution function.*1395

*That is PDF.*1407

*That is a little bit easier.*1409

*Let us go into some examples.*1412

*Here is example 1 and it is just talking about frequency graphs.*1414

*It is not actually talking about CDF or the function.*1419

*Here it says that estimate a square that falls up to 48% percentile.*1424

*Percentile is a word that we say just for cumulative frequency or cumulative relative frequency.*1430

*One thing that makes this chart easy to use is that we could just go to 48, I will use a different color and I will go all the way across and go down.*1438

*We will get a rough approximation of that score.*1454

*That score is about a little bit less than 500, let us say 480.*1459

*That is the score that certifies the 48% percentile.*1465

*If you wanted it a little bit more than 40, you could just round it to 500.*1470

*If you want it less than 48% percentile, you could round down to 450.*1474

*Here is example 2.*1485

*In problems with a normal distribution, the mean, stdev, x and the probability of x, these things are involved.*1486

*Like a puzzle, if you have 3 you could figure out 4, right?*1499

*Here we have couple of things that are missing but it gives you some of the other pieces and we have to figure out the other pieces that are missing.*1505

*I’m just going to take a look at my first line.*1514

*It says the mean is 3, stdev is 1, I have an x value, what is the probability of that x?*1519

*I’m just going to pull up a regular Excel sheet and I have just labeled it with mean, stdev x, and probability of that x.*1528

*I’m also going to write down z because often we need to find z in order to use the tables at the back of the book.*1547

*I’m going to write in z.*1556

*The mean here is 3, stdev is 1, x is 2, if we taught about that in a normal distribution picture.*1561

*The mean will be 3, stdev 1, this will be 2.*1586

*Since x is that 2, it is asking for this.*1587

*It should be about 16%, just using that empirical rule.*1593

*Let us find out exactly how much that is.*1603

*If you wanted to use the special Excel function, you could actually just use normdist because you have everything you need.*1607

*You have your x, mean, stev, and if you want the cumulative probability you would just write true which is what we want.*1618

*We will get about 16% or .159.*1634

*Another way you could do it is by finding the z score and then looking in up at the back of your book.*1640

*You could use standardized or you could find the distance between the x and the mean and divide that by the stdev to get how many stdev away.*1648

*It is 1 stdev away on the negative side.*1663

*If I did not want to use this function I could just use normdist and that is where I will put in my z score and I will get the same thing.*1667

*Those are 2 different ways that you could do it.*1683

*You could also look at that z score at the back of your book. *1685

*Let us do the second line.*1690

*Here we have the mean but we do not have the stdev but we do have x which is .1 and 1x < .1 it is at .18.*1691

*Let us sketch this out to help ourselves and I will draw it in a different color.*1713

*Here the mean is 10 but I do not know what to write here.*1723

*What I do know is around 18% which is a little more than 16%, around 18% that x is .1.*1730

*My question is what is my stdev?*1744

*What is this jump such that this 18% that is .1?*1751

*We have all the pieces that we need, one thing that might be helpful to do is find the z score because the z score formula has the stdev.*1756

*We have all the other pieces like the mean and x.*1768

*Once we know the z score then we could easily find out the stdev.*1771

*I’m just going to use my norm inverse which would get if I put in the probability it will give me the z score *1777

*or you could also look up in the table at the back of your book and look for 18% and find the z score there.*1787

*Here I find my z score is -.91.*1798

*It is a little bit closer to the mean than -1 but it is almost -1.*1804

*Once I have that then I could easily find my stdev function just by using my z score formula because the z score formula is just x – mu / stdev or /Ʃ.*1813

*What I want to solve is for Ʃ and here I could just match my Ʃ and both sides multiply and divide z by both sides and I could get Ʃ = x – mu / z score.*1838

*Then I could easily do in Excel or in your calculator or in your head.*1857

*I will just take x – mean / z score and I wil get 10.81.*1866

*That makes sense because if I went out about 10.81 that will be -.81 right here at the first stdev.*1881

*This is much smaller than that.*1893

*Here that answer makes sense.*1898

*Let us go to the third problem.*1902

*Now I’m missing my mean but I have my stdev, I have m y x which is -.6 and I know that probability where x < -.6 is 35%.*1905

*I will put in that.*1926

*Let us sketch that out just so we can check whether the answer that we get is reasonable.*1929

*Now we have an idea what this middle value is but we do know that each jump is about 3 away.*1937

*The other we cannot tell where to write that .6 we could tell by the percentage it cannot be up here.*1946

*It must be somewhere here such that this is about 35%.*1958

*It is not quite half of that half but it is a little bit more than half of this half.*1975

*The question is what is this mean?*1985

*We know at that point it is -.6.*1988

*Whatever my mean is, it got to be bigger than -.6 just because we know that this is not quite half yet and if we have the middle that is the mean.*1994

*Once again it is half way to find the z score because the z score formula has the mean in it and so it is easy to find the mean once we have the z score.*2008

*Once again I’m going to use norms in and put in my probability and I will get the z score of -.38.*2020

*That makes sense.*2036

*That is in between 0 and -1.*2037

*Let us use that in able to find our mean.*2043

*If you want you could just derive it from this again so instead of going from z, x but now we want to isolate mu.*2046

*It would be z Ʃ = x – mu and I’m going to move the mu over to this side and move this over this side.*2060

*That is what I need to do in order to get my mean, mu.*2079

*I will take my x and subtract the z score × stdev and I will get mean of .56.*2088

*That makes sense because that is bigger than .6 and since that there is a distance of 3 that would makes sense for .6 in between .55 and -2.55.*2106

*This makes sense to us so once you write the mean, the answer, you are good to go.*2137

*Now let us move on to example 3.*2146

*It is another puzzle like problem but once you know the mean and the stdev you could find out Q1 and Q3.*2153

*If you want Q1 and Q3 that could hel you figure out the mean and stdev.*2161

*Here we are missing Q1 and Q3.*2167

*Here we are missing the mean and stdev.*2171

*And here I am missing a little bit of both.*2174

*Let us get started.*2177

*Here let us start just by drawing what we are talking about here.*2180

*Here we have the mean and the stdev which is 5, 5 away on this side and 5 away on this side.*2186

*We know that at about this little score right here we know that is 34% of the curve.*2197

*Q1 wants to split it out into quartiles not 16 and 34, that is not even enough.*2207

*We know that Q1 has to be somewhere in between 1 stdev away and 0 stdev away.*2216

*We know that it means to be somewhere in there.*2225

*Not quite stdev away because we want this area which makes this 25%, makes that 25%, and same with Q3 and up here.*2229

*That is what we are looking for here.*2246

*I will show you 2 ways of doing it just with Excel.*2250

*Excel makes it a lot easier for us to do these things. *2256

*Here I have mean, stdev, Q1, and Q3.*2260

*We have the mean of 10 and stdev of 5.*2271

*One thing that is helpful is just to know what the z score is in Q1 and that is never going to change because z score is just how many stdev away.*2276

*You could think about z scores as just being a reflection of the standard normal distribution and that never changes.*2287

*The z at Q1, what is that?*2294

*That is easy to find by using the norms inv function and there you would just put in the cumulative probability you want.*2298

*That would be this area right here which is an easy 25%.*2308

*I know my z score there, it is not quite -1 it is -.67.*2314

*That makes sense.*2321

*Once we know that, then we have all the things we need in order to find the raw score at Q1 or what we call x.*2323

*We could put in the mean + z score × stdev and because Excel always holds order of operations, it will do the multiplication before it does the addition.*2331

*That is going to be 6.63 and does that make sense?*2351

*Yes it does.*2357

*It is in between 5 and 10 and it is pretty close to 5 but not quite all the way to 5.*2359

*That makes sense that would be 6.63 and we could do the same thing in order to find Q3.*2366

*Let us find the z at Q3.*2376

*If you wanted to do this without Excel you could also easily do that too because you know that the z at Q3 if you cover 75% of that curve.*2380

*This is also .25, if you add it all up that is 75% of that curve.*2394

*You could just look that up in the table at the back of your book.*2402

*Look for .75 and then look for the z score that correspond at that point or you could find that in Excel by using norms.*2406

*It is important to have that s because if you are just looking for the z score and put in the probability of 75%.*2419

*.75 gives us .67 as the z score and that makes sense that these z scores for Q1 and Q3 precisely mirror each other.*2427

*They are just the negative and positive versions of each other.*2438

*For Q3, we could just use the mean and add how many stdev away you want to go.*2442

*That is z × stdev and that gives us 13.37.*2453

*If we look on this side it is not quite 15 but it is closer to 15 than it is to 10.13.37.*2460

*That makes sense.*2471

*I will write that in here 13.37 and this is something like 6.63.*2472

*That is finding Q1 and Q3.*2485

*There is yet another way that you can do it in Excel and this is going to be a super short cut.*2489

*You could use the norm inv function because you have the probability .25.*2496

*You have the mean and the stdev.*2505

*That will give you the z score.*2510

*You could also use that for finding Q3 by using norm inv where you put in the probability and I will spit out the raw score.*2513

*The probability is 7.75, mean is 10, stdev is 5, and once again we get the same thing.*2522

*For a lot less work you do not have to go through the z score method.*2535

*The z score method is helpful just because you could also use the back of your book and maybe on your test who have Excel.*2538

*That is a good and helpful thing to know.*2547

*Now let us move on to the second problem.*2551

*The second row the problem has changed a little bit.*2555

*We have the same curve but we know that we do not know the mean, what that jump is, but we do know this.*2560

*We know here the score is 120 and we know here the score is 180.*2575

*I’m just going to show you a quick short cut here but it is very reasonable shortcut.*2585

*We know that the mean has to be in the middle of these 2 numbers.*2589

*Those 2 numbers are are mirrors of each other.*2594

*They are exactly 25% away on this side and 25% away on this side and the normal distribution is perfectly symmetrical.*2597

*We k now that the mean has to be the point in the middle.*2603

*There is only one point that is precisely in the middle of those 2 numbers and we could easily find that by taking the average of 120 and 80.*2608

*Between 120 and 80.*2619

*I could just say take the average of these 2 numbers or you could alternatively add 120 to 180 and divide it by 2 and we would get 150.*2622

*150 looks about right.*2638

*That looks like where it should fall.*2642

*The question now is what is the stdev?*2647

*One way you could easily do this is we know the z score and we could use it to figure out the stdev.*2653

*I do not have to find the z score again.*2667

*I’m just going to use these.*2669

*Now let us think about the formula for stdev.*2672

*If you remember from the previous problem, it is just going to be whatever x in.*2675

*I’m just going to use Q1 as my x – mean / z score.*2684

*I will get 44.48.*2691

*Let us see if that makes sense to us.*2695

*If we are at 150, if we go out 44.5 then that should give us about 105 or 106.*2700

*If we go out that far that makes sense because 120 falls in between that and 150 but it is a little bit closer to the 105 than it is to the 150.*2726

*If we go out on the other side it will be 194.5.*2741

*Once again it makes sense because 180 is pretty close to it but not all the way up there.*2748

*Last problem in this set.*2754

*Here we do not know the mean but we do know the stdev, the jump.*2760

*This is a jump of 10.*2770

*We know Q1.*2772

*Here is Q1 and that is 100.*2776

*We know that the mean has to be greater than 100.*2784

*We do not know exactly how much greater but we know it is greater than 100.*2790

*It cannot be 110 because you are not going 1 stdev out, you are going less than 1 stdev out.*2797

*Let us see if we could figure out this strategy.*2806

*We could find the z score very easily but we already know it.*2810

*Using the z score we could find the mean.*2814

*Once we know that we could find Q3.*2818

*I will move this up here.*2821

*Here we do know the mean but we do know the stdev.*2829

*We know Q1 and we know the z score at Q1.*2833

*Using that z score in Q1 I’m just going to go ahead and find my mean.*2838

*In order to find the mean, if you remember from the previous problem it is just x – stdev × z score.*2844

*The mean is at 106.75.*2863

*That makes sense.*2873

*It is not quite 110 but it is in between there and once we know that mean 106.7 then we could easily use that in order to find Q3.*2876

*Q3 we could use norm inv and put in probability, mean, and stdev, or you could use the z score in order to find Q3.*2909

*That makes sense 113 because if the mean is 106 or 107, then going 10 out would be 116.7 and that is too far out.*2932

*113 is perfect for Q3.*2952

*That is example 3 and notice that it just takes a little bit of reasoning to get around some of these things.*2956

*Let us go to example 4.*2972

*The miniature cars in an old town is 12 years old and the stdev is 8 years, what percentage of cars are more than 4 years old?*2975

*One thing that helps me is if I draw a little distribution to help me out.*2984

*The mean is 12, stdev is 8, what percentage of cars are more than 4 years old?*2992

*There is implicit issue here.*3024

*Let us say we go another 8 out that would mean we are at -4.*3031

*Can a car be -4 years old?*3034

*Let us think about this.*3037

*It looks like maybe in somebody’s head or in plans.*3040

*-4 years old is hard to think about.*3047

*When we think about this distribution we want to cut it off at 0 because cars just are not -1 years old.*3052

*It starts at 0.*3068

*If we think about where 0 is, that is right in between there.*3070

*When we think about it in terms of z score, the z score is 0, this is -1 and this is about -1.5.*3076

*We are thinking about we do not want to count these cars because they do not exist.*3087

*Thankfully our question is what percentage of the cars are more than 4 years old.*3099

*Our question is a trick question to help us think about this issue.*3107

*We are asking for this but remember percentage is always what is that compared to the whole.*3117

*The whole is a little bit different here.*3125

*The whole is not this whole curve because this part does not count.*3128

*The whole is actually this part.*3135

*It is asking what is the blue part in proportion to the red part?*3144

*Tricky question.*3150

*This takes a little bit of thinking.*3152

*One thing probably we want to do is figure out the proportion of cars where age is greater than 4 years old and divide that by the proportion of cars where h > 0.*3155

*That we could easily do by using z scores.*3181

*We could take the z score or p(z score) > -1 / p or the z score > -1.5.*3184

*You could do this by looking these probabilities at the back of your book or we will find these probabilities in Excel.*3202

*Here is p where z > -1 and remember because we want the greater side, we have to do 1 – the functions here *3216

*because Excel will give us the part on the negative side.*3235

*Instead of the greater than side it will give us the less than side.*3240

*I could use normsdist where we out in the z -1 and then it will split out the probability but it will split out the less than probability.*3243

*Here I want to put in the 1 – normsdist and this should be greater than 50%.*3258

*It should be 80 or something percent.*3266

*Let us find the probability where the z is greater than – 1.5.*3271

*I could use that same function normsdist of -1.5.*3281

*Once again because I want the greater than part, I want to use my 1-.*3289

*Once we have that then we could get the proportion what percentage of cars is here over here.*3296

*That would be this over that.*3306

*That is 90% of cars.*3314

*This will be 90% of cars.*3318

*This seems to be a little bit of a tricky problem.*3322

*It does not look tricky at first but watch out for things like this whether it is a cut off at 0.*3326

*You cannot have a negative age in this case.*3333

*Watch out for these problems.*3339

*That is it for www.educator.com.*3342

0 answers

Post by Mohammed Alam on August 6, 2014

Useful lecture series. Good instructor.

0 answers

Post by Manoj Joseph on June 17, 2013

I am finding difficult to make sense this session.

0 answers

Post by Robin Dorsey on October 20, 2012

This example assumes that quarterly results with have a normal distribution...why is this a reasonable assumption?

0 answers

Post by Kamal Almarzooq on January 23, 2012

too much depend on excel makes your teachings less useful to me :(