Example: Two Sample t-Test

January 31, 2012
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(This article was first published on ALSTAT R Blog, and kindly contributed to R-bloggers)

The recovery time (in days) is measured for 10 patients taking a new drug and for 10 different patients taking a placebo. We wish to test the hypothesis that the mean recovery time for patients taking the drug is less than for those taking placebo. The data are:

     With drug: 15,10,13,7,9,8,21,9,14,8
     Placebo: 15,14,12,8,14,7,16,10,15,12

A. Perform a hypothesis testing under an assumption of normality and equal population variances.

Solution:

Codes:

Output:

Interpretation: We lack the evidence to reject the null hypothesis with p-value at 0.3002, hence the true difference in mean recovery time for patients taking drug is either equal to or greater than those taking placebo.

B. Perform a hypothesis testing under an assumption of normality and unequal population variances.

Solution:

Codes: (Using the defined data above)

Output:

Interpretation: Still, we lack the evidence to reject the null hypothesis at p = 0.3006, and so the true difference in mean recovery time for patients taking the drug is either equal to or greater than those taking a placebo.

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