Dr. Cortez determined that the mean level of depression for both experimental groups was the same. However, it is important to not only look at the central tendency of both groups, but also the or degree to which they differ from each other.

Variability

Explanation:

Variability is the trait that displays variation occurring in sample of experiment and results .Variation can be present in experiment as samples may differ, condition may differ,different measuring aspect, etc.

According to the question, variability is the factor that should be considered to determine degree of difference present between data and how they are separate from each other in terms of sample structure, measurement , rate, influence etc even though it is producing same results.

the 4th option is the answer