2 sample tests CH 11 Flashcards
When we are looking at differences of proportions, what is the sampling distribution a distribution of?
A PILE OF DIFFERENCES (p hat - p hat). You have to imagine taking a a pair of samples, say.. Of girls and boys, subtracting phat girl-phat boy, and then writing that difference down. Do this over and over again, and you will have a list of differences. Now make a histogram of that list of differences, and that is your sampling distribution. It is an imagined distribution of an TON differences from a TON of samle pairs.
Where did the s.d. of differences of proportions that is on the formula sheet come from?
pythag of stats.. Adding variances! From the square root of the added variances of the the sampling distributions of the 2 proportions
How do you pool with 2 proportions?
COMBINE HANDFULS: (X + X) / (N + N)
When you are doing PAIRED or MATCHED or BLOCKED tests.. What are you finding?
The average difference.. You are doing 1 sample procedures on a NEW THIRD LIST OF DIFFERENCES
When is the only time you pool you pool? (usually)
Hypothesis tests for 2 proportions? when you are assuming they come from the same bucket?
What is difference between 2 Samp T test and a PAIRED T Test
In a two sample T test you are comparing TWO SAMPLE AVERAGES to eachother. You find the average of one sample, and then the average of another sample. In a PAIRED T test you are pairing the data and subgtracting to make a THIRD LIST of differences, then doing one sample procedures on the third list. they are paired. So you find each individual BEFORE-AFTER and take the average of all of those differences. You do ONE SAMPLE T TEST on it because you really have one mean. You just the average of the difference list.
when do you pool with means?
You don’t have to. But if you become a pro, and can prove equal variances, then you can get more precision in your results by increasing your deg freedom.
How do you find df in 2 samples?
USE CALCULATOR. you have to run an interval or a test on your TI and read the output (unless you want to use the equation.)
What is a 2 sample t interval?
You are trying to catch a true difference. Like the true difference in heights between men and women. You say “I’m 90% confident that men, on average, are between 5cm and 12cm taller than women”
when is data “paired”
when you have 2 measurements on the same subject (or matched subjects). Often BEFORE and AFTER measurements. OR?.. When you pair similar subjects and compare one to another.
What is advantage of pooling?
Pooling allows you to increase your sample size? sort of, increasing DF, allowing more precise predictions
how are 2 samp t and paired t different?
2 samp t you are loooking at a difference between 2 averages, with a paired test, you are looking at an average of a bunch of differences (generally 2 measurements on just ONE sample).
What is a 2 sample t interval?
You stand at your difference of sample means and reach out and try to catch the true difference of means in the population.
If you were going to pool with means (t), when would you?
When you have reason to believe the variances of both populations are equal.
how can you decide the right test? What are the 3 questions?
1 or 2 samples or PAIRED?
Proportions (z) or Means (t)?
Test or Interval?
