In a cohort study:

**Risk is based on proportion of persons with disease = cumulative incidence**

The concept of **risk** reflects the proportion of persons experiencing the event or outcome or disease.

**Risk ratio** = ratio of 2 cumulative incidence estimates = **relative risk**

If the concept of **risk** reflects the proportion of persons experiencing the event or outcome or disease,

- it follows that two cumulative incidences are needed for a
**risk ratio**or**relative risk**.

- Relative measure gives better sense of
**strength of an association**between exposure and disease for inferences about causes of disease

- The strength of association is one of the criteria considered in assessing causality in the relationship between an exposure and a disease.
- So ratio measures are more useful in making inferences about the causes of disease.
- Ratio measures are also given by the most commonly used multivariate analyses such as logistic regression and proportional hazards regression.

In practice many risk factors have a relative measure in the range of **2 to 5.**

In the table below it looks like a **ratio of about 3**, treating more than 3 months really does make a difference,

- but because TB recurrence is a relatively
**rare event**in treated patients, the**absolute difference of 2.6%**is not so impressive. - The
**absolute measure**is important when cost effectiveness is being evaluated.

**If incidence is very low, relative measure can be large but difference measure small.**

This is an example (below) from an outbreak of gastrointestinal illness of a risk ratio from cohort data.

- We have equal follow-up on everyone in the cohort.

Because the follow-up is short and identical for everyone, the risk ratio is just the ratio of the proportion

- With disease in the exposed group (those who ate the potato salad) and
- With disease in the unexposed group (those who did not eat the potato salad).

So the RR=11 is taken as **strong evidence for assigning causality to eating the potato salad**. It is highly likely that the potato salad caused the outbreak of gastroenteritis.

Follow-up in these cohorts, which are not very short term outbreak investigations, have differing amounts of follow-up time on the subjects.

**The risk of the event has to be estimated:**

- in the exposed and
- in the unexposed group

- using a method like the Kaplan-Meier or the life table.

In the Kaplan-Meier analysis of the survival in two groups (below), you have to choose a point in time.

**For Example:** At 6 years, % dead in low CD4 group = 0.70 and in high CD4 group = 0.26.

**Risk ratio at 6 years = 0.70/0.26 = 2.69**

As you can see from inspecting the curve, the risk ratio will be different for different points in time.

- If one point in time is selected, then the risk ratio becomes the ratio of the two proportions failing (or surviving, if you prefer) at that point in time.
- IN reporting Kaplan-Meier results you
**must always specify at what amount of follow-up time**. - This applies to the risk ratio as well.

NOTE : **Risk ratio would be different for different follow-up times**.

Topic revision: r21 - 08 Mar 2011 - 15:14:36 - MaryBanach

Copyright &© by the contributing authors. All material on this collaboration platform is the property of the contributing authors.

Ideas, requests, problems regarding CTSPedia? Send feedback

Ideas, requests, problems regarding CTSPedia? Send feedback