## Odds Ratios

Odds ratios are similar to relative risk. The calculation of relative risk however, requires knowledge of the absolute risk for the groups being compared. This requires data from a prospective study. When groups are evaluated retrospectively another measure of risk must be employed. In this circumstance, odds ratios are calculated. This is done by calculating the odds of disease for each of the two groups and then dividing to obtain the odds ratio.

For example, 1000 individuals are selected based on the individuals having a low spine bone density. The observation is made that 100 of these individuals have a spine fracture. In another group of 1000 individuals who are picked on the basis of having good spine bone densities, only 5 are observed to have a fracture. What are the odds of having a fracture for an individual in either group and what is the odds ratio of the low BMD group compared to the high BMD group?

The odds for fracture in either group are found by dividing the number of individuals with fracture in the group by the number in that group which has not experienced a fracture. Therefore, for the low BMD group the odds are:

The odds for the high BMD group are:

The odds ratio for the low BMD group compared to the high BMD group then is:

The interpretation of this odds ratio is that fracture is 22.2 times more likely in the low BMD group compared to the high BMD group. These values were not taken from the medical literature and are used for the purposes of illustration only.

For uncommon diseases, the relative risk and odds ratio will be very similar. For common diseases however, the odds ratio will be greater than the relative risk.

## Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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