Statistical Measures

Ratios, Proportions, Rates

Rates:

• The third type of frequency measure used with two-category (dichotomous) variables is a rate.
• Rates have the added dimension of time. Rates measure the occurrence of an event in a population over time.
• The basic formula for a rate is:

 

 
• Rates are always specific to a particular population. They reflect groupings of people based on time, place and person.
• Time: a specific year, month, week, day or hour
• Place: country, state, county, township, school, institution, area
• Person: age, sex, membership in some group or class
• Rates take into account the size of the population, so comparisons can be made across different population groups.
• By using rates instead of raw numbers, the occurrence of disease in one group can be fairly compared with another.

• Example: males with females; one county with another; Missouri with Arkansas or the US.
• To calculate a rate, we must have an estimate of the population at risk during a specific time period for the denominator.
• Ratios and proportions do not require this.
• Earlier, we calculated ratios and proportions of EMS cases without knowing the number of people at risk of EMS.
• Rates may be harder to get, because accurate denominator data may not be available for small, localized population groups.