When we are interested in studying a quantitative (numerical) random variable relating to individuals in a population, the mean is a natural parameter to use summarize the values of the variable for those in the population. If, however, we are interested in studying a qualitative (non-numerical) random variable, the mean cannot be calculated. It the qualitative variable in question is simplyr a \"Yes\"r or \"No" answer as to if a particular individual has a certain trait or Characteristic, then a the population proportion is likely to be the most useful parameter for describing the random variable