I'm trying to understand how #1 is rate and not proportion since I thought they compare pregnant to not pregnant freshman.

Which type of measure of disease frequency best describes each of the following scenarios? Incidence or prevalence and indicate whether it is a rate or proportion. A ratio is comparison of numbers. A proportion is a type of ratio that relates a part to a whole. Numerator is part of the denominator. Rates are a special type of ratio that incorporates the dimension of time into the denominator. Ratios, Proportion, and Rates (bu.edu) 1. Number of freshman girls who become pregnant over the course of their high school years. Incidence (rate)