This tutorial demonstrates step-by step methods for running a paired samples hypothesis test in SPSS. This type of statistical test is variably referred to as related sample, matched pairs or repeated measures testing. The test essentially tests for a difference in a variable under different conditions.
For example consider tip size. Do you tip differently depending on whether your server is male or female? We might expect thaat all things being equal, there should be no real difference on average. Well we can test this hypothesis using paired samples methodology by taking a sample of tips at various times of the day for male and female servers. The variable being tested is the tip and the condition is the server gender.
This type of test is often used in before and after trials for medical treatments. In other words if you want to test a weight loss drug, you might get a sample of people and weigh them before taking the drug and compare this pre-drug weight to the same person's weight after some time on the drug.
In any case the null hypothesis will always be the there is no mean difference in the variable under the different conditions.