A nonprofit organization is determining which existing donors will donate again for a current campaign. The organization decides to look at its donor database for all donors who have donated two or more × over the past five years. Which measure should be used to determine the probability of receiving a donation from an existing donor over time?

R-squared is a statistical measure that represents the proportion of variance for a dependent variable that's explained by an independent variable or variables in a regression model. While it indicates the strength of a relationship, it does not directly measure the probability of individual future donations, making it unsuitable for the organization’s needs.

The F-statistic is used to compare statistical models and determine whether the variance between two or more groups is significantly different. It is primarily used in the context of ANOVA or regression analysis and does not provide a direct measure of an individual donor's likelihood of donating again, thus rendering it inappropriate for this scenario.

The median is a measure of central tendency that indicates the middle value in a dataset. While it can provide insight into the typical donation amount, it does not account for variability or the likelihood of repeat donations over time. Therefore, it fails to deliver the nuanced probability assessment required by the organization.