Simplest form of control is comparison. Experiments should compare two or more treatments in order to avoid confounding the effect of a treatment with other influence. Creates groups that are similar, except for chance variation. Table of random digits can be used to choose treatment groups. A block is a group of experimental units or subjects that are similar in ways that are expected to affect the response of the treatments.
Treatment is assigned randomly within similar blocks.
Blocking are a form of control. A common form of blocking for comparing two treatments. One of pair receives treatment, the other a placebo. Pairs observed at a later time to see if treatment had effect. Purpose is to see if treatment improves test performance.