Measure of Association - Risk Difference

Lead Author(s): Jeff Martin, MD

Why Use Risk Difference?

Risk difference gives an absolute measure of the association between exposure and disease occurrence: Difference or absolute measures are important because they indicate how much disease eliminating the exposure (or providing the treatment) would prevent.

Definition of Risk

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.

Most Useful Application of Absolute Difference

Reciprocal of Absolute Difference OR ( 1/difference) This is one of the most useful applications of the absolute difference, the number needed to treat to prevent one case of disease. From this relationship one can see how the absolute difference can be used in cost effectiveness analysis.

TB treated with rifampin example: (above) In this example, we might ask what are the cost implications of treating 38.5 persons an extra 3 months to prevent one case of TB? Of course, the two costs may not be the only the consideration.

In this example, TB is a contagious disease and allowing one case of recurrence by under-treating could lead to additional infections and other cases of TB.

Study Reporting Risk Difference

In the table below Stiell provides the following information:


Risk difference for return to spontaneous circulation (labeled Absolute Increase) = 0.051; Risk difference for admission to the hospital (labeled Absolute Increase) = 0.037; Here is an example of a in the New England Journal of Medicine that reported risk differences rather than risk ratios.

This shows two of their findings, the absolute increase in the percent of patients suffering heart failure having a return to spontaneous circulation and the absolute increase in the percent of patients having admission to the hospital.

Because this was an attempt to evaluate a public health intervention of making advanced life support available to the emergency responders for all such cases, the absolute percent of cases, rather than a ratio measure, was of primary interest. (If a risk ratio had been supplied for return to spontaneous circulation, it would have been around 1.5.)

The two phases are before and after advanced life support became the standard for emergency responses in this location. The absolute difference is low, but the percentages were low on both phases.


Stiell, I. G., Wells, G. A., Field, B., Spaite, D. W., Nesbitt, L. P., De Maio, V. J., et al. (2004). Advanced cardiac life support in out-of-hospital cardiac arrest. N Engl J Med, 351 (7), 647-656.