A variable (factor) that cannot be randomized. The experiment is usually run in blocks for each level of the blocking variable and randomization is performed within the blocks.

A categorization variable for information within a dataset that is used to control, test, or manipulate the distribution and statistical results. These variables should be observed, reliable, and unchanging. For example, a dataset can be blocked based on name, date, race or gender. It's a way of viewing the data from a preferred reference point within a random collection of information.