Absolute risk reduction , calculation

Recent publications on major clinical trials whose implications will involve a recommendation to change clinical practice have included summary statistics that quantify the risk of benefit or harm that may occur if the results of a given trial are strictly applied to an individual patient or to a representative cohort. Four simple calculations will enable the non-statistician to answer the simple question How much better would my chances be (in terms of a particular outcome) if I took this new medicine, than if I did not take it . These calculations are the relative risk reduction, the absolute risk reduction, the number needed to treat, and the odds ratio (see Box 6.3). 

While the relative risk is a standard statistic that can be used to compare treatments, it can be difficult to understand and to relate to practice. For example, although the relative risk of 3.7 that was calculated above indicates that Drug A is associated with nearly four times the risk of cure compared with Drug B, this gives no indication of the practical implications. For this reason, effects are often quoted as the Number Needed to Treat (NNT). The NNT is calculated as the reciprocal of the absolute risk reduction (ARR). In the example in Table 3, the NNT refers to the number of patients who need to receive Drug A before an additional cure is likely to occur. 

The primary endpoint was to prevent cardiovascular (CV) death, myocardial infarction (Ml), or stroke. To calculate the number needed to treat (NNT), first calculate the absolute risk reduction (ARR). This is... 

Mayne TJ, Whalen E, Vu A. Annualized was found better than absolute risk reduction in the calculation of number needed to treat in chronic conditions. Journal of Clinical Epidemiology 2006 59 217-223. 

If both frequency and consequence values are calculated and reported on an absolute basis, then they may be reported graphically in combination with one another (Chapter 3), or simply as the product of frequency and consequence. Table 5 contains some examples of typical risk estimates (frequency and consequence products). Based on absolute risk estimates, you can decide whether the risk of a specific activity exceeds your threshold of risk tolerance (risk goal). If so, analysts can estimate the reduction in risk, given that certain improvements are made, assumptions changed, or operating circumstances eliminated. 

Individual and societal risks are different presentations of the same underlying combinations of incident frequency and consequences. Both of these measures may be of importance in assessing the benefits of risk reduction measures or in judging the acceptability of a facility in absolute terms. In general, it is impossible to derive one from the other. The underlying frequency and consequence information are the same, but individual and societal risk estimates can only be calculated directly from that basic data. A high societal risk does not necessarily mean any one person has a high individual risk and vise-versa. These are two very different criteria and should be treated separately. 

Then, the absolute reduction in frequency, consequence, or risk can be calculated and compared to the cost of implementing the improvement, allowing you to determine whether the change represents the best use of resources to improve safety. 

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