Proportion

A proportion rescales a count so that groups of different sizes become comparable, and because it is a part of a whole it can never fall below 0 or rise above 1. In symbols $\hat{p} = x / n$, where $\hat{p}$ (p-hat) is the sample proportion, $x$ is the count in the category, and $n$ is the sample size; the population version is written $p$. For example, if 45 of 300 students walk to school then $\hat{p} = 45 / 300 = 0.15$, or 15 percent. Proportions carry no units, which is what separates them from rates such as 7 cases per 1,000 people per year.