If the risk rate is equal to one then there

is no association between exposure and the disease or health outcome.

If the risk ratio or rate ratio is greater than one then the risk

in the exposed or rate in the exposed is greater than in the unexposed.

If the risk ratio or rate ratio is

less than one, then the risk in the exposed is lower than the unexposed.

For absolute measures of association, i.e.,

differences, if the risk difference is equal

to zero, then there is no association, i.e.,

the risk is the same in the both groups.

If the risk difference is greater than zero, then the

risk in the exposed is greater than in the unexposed.

And if the risk difference is less than zero, then the risk in the exposed is less

than in the unexposed.

Note that the null value for differences is zero

while for the ratios the null value is one.

Let's preface the topic of which measures of association

are commonly found with different types of study design by

first noting that all the measures of association we

have covered can be estimated in the cohort study design.

However, some of the other study designs

are not able to directly calculate risks and rates as you can in the cohort.

So, here is a table illustrating the measures of association

that can be commonly used for different types of study designs.

Note that prevalence and odds ratios and

differences are more commonly found with cross-sectional and

case-control studies, while risk and rate ratios and

differences are more commonly used with cohort studies.

Risks and rate ratios cannot be directly

calculated from case control and cross sectional studies.

This concludes the segment on interpreting measures of association.