Lesson 7: t-test

Student's t-Tests

http://www.physics.csbsju.edu/stats/t-test.html

We use this test for comparing the means of two treatments, even if they have different numbers of replicates. In simple terms, the t-test compares the actual difference between two means in relation to the variation in the data (expressed as the standard deviation of the difference between the means.

Table of t-statistics

Choosing an analysis to compare two groups

Paired or unpaired test?

When choosing a test, you need to decide whether to use a paired test. Choose a paired test when the two columns of data are matched. Here are some examples:

You measure a variable (perhaps, weight) before an intervention, and then measure it in the same subjects after the intervention.

You recruit subjects as pairs, matched for variables such as age, ethnic group and disease severity. One of the pair gets one treatment, the other gets an alternative treatment.
You run a laboratory experiment several times, each time with a control and treated preparation handled in parallel.

You measure a variable in twins, or child/parent pairs.

More generally, you should select a paired test whenever you expect a value in one group to be closer to a particular value in the other group than to a randomly selected value in the other group.