# Risk ratio versus Rate ratio

Relative Risk is used to encompass the following:

• Risk ratio
• Rate ratio
• Relative risk
• Relative rate

Simply put, the relative risk is the risk of something in one group of people relative to the risk of the same thing in another group of people. The difference between the two groups is usually exposure status- one group is exposed, while the other is unexposed. In any case, we are computing risk/ rate keeping risk/ rate in one group as the baseline value, and determining risk/ rate in another group relative to that baseline.

It is important to note that the calculation of risk ratio/ rate ratio/ relative risk is restricted to studies where incidence can be calculated. Case-control studies yield Odds Ratios that are good estimates of the Relative Risk when the disease is rare (prevalence is less than 20%).

What if the study included more than two cities- how would the risk ratio be calculated?

The basic procedure will not change. As in the previous instance, a reference city is chosen (generally the city with least incidence of respiratory illness) and other cities are compared with it in turn.

Example:

Let us assume that a third city (C) having air pollution intermediate between city A and city B was included in the study. Now we can depict the cities as having high (A), medium (C) and low (B) air pollution as shown below:

The Risk Ratio is calculated keeping the lower incidence as reference (baseline):

The risk ratio for high versus low air pollution is 0.4/0.1 = 4.0

The risk ratio for medium versus low air pollution is 0.25/0.1 = 2.5

The risk ratio for high versus medium air pollution is 0.4/0.25 = 1.6

These risk ratios are interpreted as described previously. A similar procedure is employed for Rate Ratios when there are more than two categories of exposure/ exposure levels.