The document discusses frequency measures used in epidemiology, including ratios, proportions, and rates. Ratios compare the occurrence of a variable between two groups, proportions express the size of one group compared to a larger group it belongs to as a percentage, and rates measure the occurrence of an event in a population over a period of time. These measures are calculated in the same way and used to describe disease occurrence and mortality in infectious disease epidemiology. The document also outlines some strengths and limitations of epidemiological study designs, such as bias, misclassification, confounding, and effect modification.