UNLABELLED: WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT: Several comparative measures (ratios, differences, or the number needed to treat) are used to express the effect of a drug or another intervention. These measures can vary in the way they are affected by the background risk measured from the reference group. WHAT THIS STUDY ADDS: This paper reviews the formulation, interpretation and limitations of measures of effect. We describe a little-known parameter, the attained effect or clinical result ratio, a positive reformulation of the relative risk difference, and suggest how available parameters can be best used to summarize results of studies of the effect of drugs. AIMS Measures to compare two drugs are often affected by the background risk in the reference group; a ceiling effect results when the background risk is small. We review measures of the effect of drugs, including a special formulation of the relative risk difference, the attained effect or clinical result ratio, that addresses background risk and ceiling effect. METHODS: Existing measures are the risk and odds ratios, the absolute and relative risk differences, and the number needed to treat. The attained effect is defined as the observed gain in success (the difference of proportion of success between the two interventions), divided by the maximum attainable gain, the maximum proportion of success one can expect. We illustrate the relationship between these measures with published results of two meta-analyses. RESULTS: In studies of the effectiveness of cell salvage, the baseline risk ranged between 8 and 95%. This variability affected the risk difference and number needed to treat, while the attained effect, with a ceiling residual risk of 2%, showed that the gain in success was half the maximum attainable gain. In studies of the effectiveness of therapy in patients infected by the human immunodeficiency virus, where the baseline risk was less variable, and there was no ceiling effect, the maximum attained effect indicated that the gain could be much smaller. CONCLUSION: The attained effect, interpreted as the proportion of effectiveness that remains to gain for future interventions, can usefully complete the number needed to treat as a clinically informative effect measure.