The number of levels of a factor or independent variable is equal to the number of variations of that factor that were used in the experiment. If an experiment compared the drug dosages 50 mg, 100 mg, and 150 mg, then the factor "drug dosage" would have three levels: 50 mg, 100 mg, and 150 mg. Be sure not to confuse the number of levels of a factor with the number of factors in an experiment. Drug dosage is one factor consisting of three levels. A factor must have at least two levels. If a factor only had one level then the effect of the factor could not be assessed.