## lgbasallote 3 years ago "All the names of a class were written on separate tickets. Ten tickets were selected after the names were jumbled inside a can...." This is an example of simple random sampling right?

1. charlotte123

Yes I believe so :D Since its random ;D

2. lgbasallote

well...i know it's random....i just don't know the specific type of random sampling

3. charlotte123

Isnt there only one type? :O do u have choices lol? :P

4. lgbasallote

it could be clustered random sampling....since they were grouped first... or maybe stratified random sampling since i don't know what it means....

5. lgbasallote

no there isn't one type and no choices

6. KingGeorge

I would probably argue that they're looking for clustered random sampling. They first divided some population up into "classes" and from this particular class, they made a simple random sampling. I see how this is confusing though. Since a clustered random sampling by definition has a simple random sampling in it...

7. vf321

Taking AP Stats, eh @Igbasallote? It's SRS.

8. lgbasallote

clustered random sampling has something to do with grouping right @KingGeorge ?

9. lgbasallote

@vf321 not taking it....it's complicated anyway why SRS?

10. vf321

One sec let me take my notes out with precise definitions.

11. KingGeorge

clustered random sampling is when you split the population into multiple groups/clusters, and then take a simple random sample. By saying this was from a "class" it implied to me that they had divided the overall population into multiple "classes"/groups.

12. lgbasallote

yes. my thoughts too

13. KingGeorge

It's that fact that confuses the question a little for me. Since any example of clustered random sampling is an example of simple random sampling as well.

14. lgbasallote

a lottery is a simple random sampling though...

15. vf321

Def 1. SRS is a sample selected from a population that ensures that EVERY possible sample of desired size has the same chance of being selected (i.e., random number generator - but, the College Board counts the shuffled names on tickets as random) Def 2. A population can be divided into non-overlapping subgroups called clusters which are internally heterogenous. Random selection of several clusters and then assessment of all internal individuals is cluster sampling.

16. lgbasallote

...so how is it not cluster nor random?

17. vf321

You are taking ONE sample from your ticket pool, and each ticket has an equal chance of being drawn, assuming you take them at the same time, and do so randomly. You are NOT dividing the class into, say, desk groups and randomly choosing entire desk groups for your sample. QED, SRS.

18. vf321

**QED was necessary for style points.

19. KingGeorge

Ah. It makes more sense now. In a cluster sampling, you also have to take a sample of each cluster. Hence, this can't be CRS, so it must be SRS.

20. KingGeorge

^^I think that's the right idea. Feel free to shoot me down if it's wrong.

21. lgbasallote

by SRS we're talking about simple? or stratified?

22. KingGeorge

Simple I think.

23. vf321

Simple of course. Stratified sampling can be thought of as homogenous clustering.

24. lgbasallote

oh..i thought SRS meant stratified

25. vf321

And stratification would result in an internal SRS for each strata.

26. lgbasallote

that was why i was confused with the definition

27. vf321

Well are you all clear now?

28. lgbasallote

what is stratified random sampling then?

29. lgbasallote

@vf321 ?

30. vf321

I told you, didn't I? "Stratified sampling can be thought of as homogenous clustering," but where each such homogenous cluster has an "internal SRS for each strata." I.e., you don't use the entire strata as your sample.

31. lgbasallote

i meant....how does it look like...i not good with understanding stat words

32. lgbasallote

you choose part of the cluster?

33. lgbasallote

or you divide a population into different clusters and then you select one cluster?

34. vf321

Stratified sampling: I want to observe high school student's attitude towards school assemblies. I know that there may be varying attitudes over grade level. Strata: Freshmen, Sophomores, Juniors, Seniors. Take a SRS (simple RS) of each strata (make sure size of SRS is proportional to grade population): 30 rand selected fresh, 20 r.s. soph., 25 r.s. juniors, 20 r.s. seniors. Sample = 30 + 20 + 25+ 20

35. vf321

This is done so that I have a sample that represents my population well. A whole SRS of the entire high school of size 95 can end up with, say, 50 sophomores, which may skew the results.

36. lgbasallote

ohhh

37. lgbasallote

but simple random sampling takes all of the population into account?

38. vf321

yes look at my definition above

39. lgbasallote

like i said..im not good with stat words...just clarifying