Requirements for applying a paired t-test

1 The column of individual changes should be consistent with a normal distribution 

The paired /-test is just a special application of the 95 per cent Cl for the mean and the requirement for normally distributed data, described in Chapter 5, applies in this case too. Just be aware that the confidence interval is calculated using the column of individual changes in weight (etc.), so it is these that need to be normally distributed. The subjects placebo and active treated weights could be skewed or bimodal or any other horrible distribution - that would not matter. We just need the changes to be normal. As previously, we do not expect small samples to form perfect classic normal distributions, but if the column of individual changes shows extreme signs of nonnormality, we should not trust a paired /-test (see Chapter 18 for possible solutions). 

The individual changes must be consistent with a normal distribution. There is no need for the original observations to be normally distributed. 

10 Sample sizes, practical significance and one-sided tests 

Chapters 8-10 explained sample size calculations, practical significance and onesided tests in detail and the point was made that these were general concepts that could be applied to a wide variety of tests. This section briefly reviews their application to the paired /-test. 

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