Number Needed to Treat, Absolute Risk Reduction, Attributable Risk, Number Needed to Harm

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Attributable Risk (AR) and Absolute Risk Reduction (ARR) are how much of the observed change in risk is due to the treatment (or exposure) being studied. Put another way AR is the amount of disease that would be eliminated if the exposure was eliminated. ARR would be the amount of disease that would be eliminated if all patients were receiving the drug. ARR and AR are essential the same thing but used in different situations. They are both calculated the same way. The only difference is that in AR the probability of disease is going down due to a treatment and in ARR the probability is going up due to an exposure or risk factor.

ARR = probability of disease among exposed -- probability of disease among the non-exposed
AR = probability of disease among those without treatment -- probability of disease among treated

I'm never able to keep the two formulas straight and the difference isn't that important so I don't even try. I just remember it like this:

ARR or AR = higher probability -- lower probability

Both Number Needed to Treat and Number Needed to Harm are 1 divided by the absolute risk reduction or attributable risk (whichever is more appropriate). The Number Needed to Treat is how many people you need to give a particular treatment to in order to have a positive effect on one person. Number Needed to Harm is the number of people that need to be exposed to a risk factor to effect one person.

Number needed to treat = 1 _ absolute risk reduction
Number needed to harm = 1 _ attributable risk

Now that you have finished this video you should check out the next video in the Biostatistics & Epidemiology section which covers the Null Hypothesis, Alternative Hypothesis & Types of Error (http://www.stomponstep1.com/p-value-n...)