Descriptive statistics describe the sample you have, it describes the behavior of a variable. Inferential statistics draw conclusions about the non-random relationship between two or more variables. Randomness creeps in between two samples: two samples taken from the same process never yield the same mean, and a discrepancy may simply be a sampling fluctuation.
A statistical test answers a single question: Is the observed difference too large to be attributed to chance? That's what distinguishes «the two machines seem different» from «the two machines are different.».
Click on the «Data Analysis» menu, then click on «Inferential Statistics.».
