An experiment tries to evaluate the effects of one or more treatments. Imagine you wish to evaluate the effect of some treatment. One natural strategy is: choose groups of subjects; apply this treatment at different levels to different groups; then see if the groups are different after treatment. For example, you might wish to investigate whether consuming a painkiller affected headaches. Different levels of treatment would correspond to different numbers of pills (say, none, one or two). Different subjects would be different people. We would then divide the subjects into groups, apply different levels of treatment (i.e. give them different numbers of pills), and record outcomes. Now we need to tell whether the groups are different.