Changes over time can be well illustrated by percent change. However, if another variable, population, is added, that percentage must be on a per capita basis to show accurate figures. For instance in the city of Palm Springs, in the year 1993, there were 1000 pets registered with the city. In 1995, there were 1100 pets. It is obvious that we increased the number of pets by 100 and the percentage of change, is an increase of 10 percent. If the population was 10000 people in 1994 and 13000 people in 1995, did we actually increase the ratio of pets to population, or decrease it on a per capita basis?
To determine that, you must divide the number of pets by the number of people in the city.