Two-sample t test
Key Questions
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I have found a nice example for you to check out on the Penn State web page.
With the final notice saying:
"Comparing two proportions – For proportions there consideration to using "pooled" or "unpooled" is based on the hypothesis: if testing "no difference" between the two proportions then we will pool the variance, however, if testing for a specific difference (e.g. the difference between two proportions is 0.1, 0.02, etc --- i.e. the value in Ho is a number other than 0) then unpooled will be used."
as a quick sum up.
here is a link to a detailed description, if you would like some more detail as to why this is.
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Conditions for conducting two sample t-test for means:
1.) Parent populations from which samples are drawn are normally distributed.
2.) The two samples are random and independent of each other.
3.) Population variances are equal and unknown.For validity of t-test we should check the equality of variances with the help of F-test for equality of variances. If the hypothesis is rejected then we cannot apply t-test. In such cases we apply Behren's d-test.
But for practical problems the assumptions 1.) and 3.) are taken to be true.