How many independent variables do you have? This is the same as asking how many sets of grouping variables do you have? Or how many sets of groups do you have?
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More Information (if you need help deciding)
Single Independent Variable (1 group variable): This could be something like eye color (brown, blue, green) OR age (young, middle-aged, elderly).
Single Independent Variable (1 group variable with a covariate): You only have one set of groups in your data, but you want to measure the difference between them in the presence of another variable (called a covariate). This means we may have selected young male, middle-aged male, and elderly males as our groups, and we want to compare their weights to see if any differences exist, but we want to control for, or remove the effects from, height to get a better sense of weight differences. Accounting for height in this analysis is called including a covariate in a model.
Multiple Independent Variables (2+ group variables): This could be something like eye color AND age, creating the groups brown eyed and young, brown eyed and middle-aged, brown eyed and elderly, blue eyed and young, blue eyed and middle-aged, blue eyed and elders, green eyed and young, green eyed and middle-aged, and green eyed and elderly. Notice there are two sets of groups (eye color and age), and we care about differences on our variable of interest (weight for example) across all combinations of the two sets of groups.