Factors are the variables that experimenters control during an experiment in order to determine their effect on the response variable. A factor can take on only a small number of values, which are known as factor levels. Factors can be a categorical variable or based on a continuous variable but only use a limited number of values chosen by the experimenters.
ANOVA and design of experiments use factors extensively. For example, you are studying factors that could affect athletic performance. You decide to include the following two factors in your experiment:
| Factor | Equipment brand | Room temperature |
| Level | A | Low (65F) |
| Level | B | Medium (70F) |
| Level | High (75F) |
Equipment brand is a categorical variable. It can only be type A or type B. On the other hand, the temperature of the room where training occurs is a continuous variable. However, in this experiment, temperature is a factor because the experimenters set only three temperatures settings: 65F, 70F and 75F.