Researchers in sonography, as well as other areas, often wish to measure the strength of relationship or association between two variables. For example, one may wish to determine if, on the average, total cholesterol level increases as age increases for adult American men. However, there are a very large number of measures or coefficients (i.e., a number that indicates the strength of the relationship between two variables) from which to choose. It is not infrequent to find a researcher selecting an incorrect coefficient to measure a given association, thereby possibly rendering a false or misleading conclusion. The choice of the proper measure of association is based on, among other things, the characteristics of each of the two variables involved. This article enumerates every case that can be encountered by the researcher and provides an appropriate measure of association that can be used.