For more information, please see full course syllabus of Statistics

For more information, please see full course syllabus of Statistics

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## Transcription

## Related Books

### Sampling Methods

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 0:00
- Roadmap 0:05
- Roadmap
- Biased vs. Unbiased Sampling Methods 0:32
- Biased Sampling
- Unbiased Sampling
- Probability Sampling Methods 2:31
- Simple Random
- Stratified Random Sampling
- Cluster Sampling
- Two-staged Sampling
- Systematic Sampling
- Example 1: Which Type(s) of Sampling was this? 8:33
- Example 2: Describe How to Take a Two-Stage Sample from this Book 10:16
- Example 3: Sampling Methods 11:58
- Example 4: Cluster Sample Plan 12:48

### General Statistics Online Course

### Transcription: Sampling Methods

*Welcome to www.educator.com.*0000

*We are going to be talking about sampling methods.*0003

*Previously we talked a lot about bias sampling methods and the different kinds of problems that those have.*0007

*Now we are going to talk about unbiased sampling methods, sort of the good guys.*0012

*These methods are all called probability sampling methods.*0018

*As you could tell the biased sampling methods like convenience sampling and judgment sampling methods those are called nonprobability sampling methods.*0022

*Biased sampling or non-probability sampling methods.*0032

*Those kinds of sampling methods are ones that we already talked about sampling from volunteers, convenience sampling and also judgment sampling.*0047

*These kinds of sampling methods, the method itself is not that great.*0056

*You might in depth with representative of sample, but the problem is the method is not that great and because of that there is always the suspicion of biased.*0062

*In unbiased sampling or probability sampling the idea is that the sampling method itself is robust and trustworthy.*0075

*It is not a guarantee that you will end up with a representative sample, but the method is unbiased.*0089

*So because of that we are more likely to trust this kind of sample than one of these kinds of sampling methods.*0096

*The basic principle behind all unbiased sampling is the idea of trusting in randomness because you are going to be using randomness.*0104

*This means that humans are biased, we do not know what kind of biases we are going to introduce.*0116

*We are not going to be using judgments.*0123

*We are not going to be just using convenience or whatever is around us.*0125

*We are not just going to sample from volunteers.*0129

*We are going to sample randomly and that does not mean randomly as I do not know what is going on.*0132

*We are going to sample in a probabilistic way such that we get this uniformly random sample.*0140

*There are five different kinds of probability sampling methods that you should know about.*0154

*The first one is this, pretend these are all different kinds of people, lakes, or animals or items or whatever or ice cream pints in our population.*0159

*In a simple random sample what you would do is get a directory or a list of all person in the population.*0175

*By using random numbers, number of every person of and then by picking random numbers you are going to pick a subset of that population.*0190

*You will number everybody in 1 through 1000 and then pick random numbers either from a random number generator and end up with a random sample.*0204

*This is a pretty difficult kind of sampling method because you have to list the entire population first.*0232

*This is often only used in a limited number of cases.*0242

*The next is stratified random sampling and this is actually a little closer to judgment sampling,*0248

*sort of like a mix of judgment sampling and simple random sampling.*0255

*First what you do is you divide your population into meaningful strata, into meaningful groupings that are somehow important to your study.*0260

*If size is important then you might separate by size first.*0270

*If gender is important, if the city is important, if cost is important, whatever it is you stratify, you create strata first, groupings first*0274

*Once you have created these groupings then you use simple random.*0288

*Then you list everyone in this little strata, use random numbers in order to pick a subset right and then you do that for this strata and you also do it for the strata.*0293

*In that way you make sure to have a little bit of each strata in your sample.*0316

*Cluster sampling is a way of picking clusters first and then you take everybody in there as your sample.*0326

*One common place that this is used is often in things like sampling from schools.*0336

*Sampling from schools is often easier than sampling students.*0348

*It is easier to list all of the elementary schools in your district than to list all of the elementary school students in your district.*0352

*You might list all of the elementary schools and then you use simple random sampling to sample one of these clusters*0361

*or two of these clusters or however many you want.*0370

*Right and then you take everybody in that cluster.*0373

*Maybe you will study these two elementary schools and study everybody in them.*0377

*Another kind of cluster sampling is called 2 stage sampling.*0385

*2 stage sampling is first doing cluster sampling and then next once you have the cluster then you do simple random sampling.*0390

*You start with cluster sampling.*0401

*You pick the clusters randomly then once you have the clusters you use simple random where you list everybody in your cluster*0403

*and then use random numbers in order to pick people in that sample.*0419

*We might pick just those individuals, not everybody in that cluster.*0428

*That is the difference between cluster sampling and 2 stage sampling.*0433

*Cluster sampling only picks the cluster and then you are done.*0437

*Here you put the cluster once you have the cluster then you do simple random.*0440

*Finally, we have something called systematic sampling.*0447

*Here this may be used at a factory and they might choose every case item to some variable and every case items to be picked for sampling.*0450

*Let us say at Ben & Jerry's ice cream factory and they pick every 230th pint that comes off the factory line to be picked for quality control.*0467

*For this example, maybe I will pick just 10.*0485

*1, 2, 3, 4, 5, 6, 7, 8, 9, 10 so they will pick that one.*0487

*1, 2, 3, 4, 5, 6, 7, 8, 9, 10 they will pick that one.*0493

*They will systematically go through and pick every 10th pint or 200 pint or 1000 pint.*0498

*That would be systematic sampling.*0508

*Let us go into examples.*0513

*Example 1, a pharmaceutical company sampled their drug to see whether the potency of the pills match the potency claimed on the bottle.*0518

*Every 20th box off the factory line was pulled out and 3 bottles were picked randomly and then 10 pills were sampled at random.*0527

*Which types of sampling was this?*0537

*First there is systematic sampling right.*0542

*And that is taking every 20th box, equals 20 here.*0552

*Every 20th box of the factory line was pulled out and then 3 bottles were picked randomly.*0557

*That cluster.*0564

*But then 10 pills where sampled randomly from those bottles.*0568

*That is two stages.*0583

*That would be both cluster plus simple random.*0584

*By sampling the 10 pills randomly we do not just mean that they just shoot out 10 their hands*0592

*instead they would have to take those however many pills are in that box to get random numbers.*0598

*Take out 10.*0610

*That is how they need to choose 10 in order to do simple random.*0612

*Example 2, supposed you want to estimate the average number of capital letters per printed line in order to sample the readability of a book.*0619

*Describe how to take a two-stage sample from this book.*0628

*First we need to figure out what the clusters are.*0632

*You can have clusters B chapters, you can have cluster B lines, paragraphs, or sentences.*0636

*You do not have to decide what the clusters are.*0643

*You would have to pick the samples in order to and then you have to pick another subset of those clusters in order to do simple random.*0648

*Maybe in 2 stage the first way to cluster and so maybe I will pick pages.*0664

*Pages will be my cluster and so what I will do as I will have all the numbers of the pages and then generate 10 random numbers and then pick those 10 pages.*0674

*First I will pick the pages then I use simple random in order to pick which lines and then I will count the capital letters.*0687

*Then pick lines and then I will count capital letters in those lines and get an average.*0700

*And then I can look at averages, standard deviations, those kinds of stuff once I have that data.*0712

*Example 3, the American pet lovers association has a directory of 10,000 members.*0720

*You want to send out a survey to 1000 people randomly.*0725

*How would you go about doing this?*0729

*Well in this case we have a directory so we could use just the simple random if we wanted to.*0731

*I do not know if you wanted to do it and maybe you want to have strata sampling, two separate people*0740

*into even numbers of dog lovers and cat lovers and bird lovers whatever else.*0748

*One we could just start off is just doing simple random.*0754

*You take all 10,000 of those numbers, number them all, generate 1000 random numbers and send surveys out to those 1000 people.*0758

*Example 4, to see how well US third graders do on arithmetic test, you will choose a simple random sample*0770

*of children enrolled in the third grade and give them math test .*0777

*Describe how to change this into a cluster sample plan.*0780

*When my cluster sample be a more practical plan.*0784

*Generating a list of all the children enrolled in third grade in the United States may be pretty hard.*0789

*It might be easier although not easy to make a list of districts first and then out of sampling those districts*0795

*then we might want to sample schools and then sample third-graders.*0810

*If we just wanted to do one step cluster sample plan maybe we will make a list of all elementary schools*0817

*then we will give all of those elementary schools a number and then randomly generate however many schools we need to participate.*0831

*Then out of those schools we will ask all of the third-graders at those schools to please take this exam.*0841

*Cluster sample might be more practical plan because it is slightly easier to get a list of all the elementary schools than it is to get a list of all the third-graders.*0849

*That is it for the probability sampling.*0860

*Thanks for using www.educator.com.*0864

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