A follow-up study was conducted to assess the effect of circumcision on the incidence of penile carcinoma. A large cohort was investigated. The rate in uncircumcised males was 8/1000, and the rate in circumcised males was 6/1000. The investigators concluded that circumcision may be markedly effective in reducing the incidence of the disease in the population. According to the study results, how many patients should be circumcised to prevent one case of penile carcinoma?
A)100
B)250
C)500
D)1000
E)5000

Question

A follow-up study was conducted to assess the effect of circumcision on the incidence of penile carcinoma.  A large cohort was investigated.  The rate in uncircumcised males was 8/1000, and the rate in circumcised males was 6/1000.  The investigators concluded that circumcision may be markedly effective in reducing the incidence of the disease in the population.  According to the study results, how many patients should be circumcised to prevent one case of penile carcinoma?
A)100
B)250
C)500
D)1000
E)5000

Quizplus · Accepted Answer

Explanation: The "number needed to treat" (NNT) is the number of patients who need to be treated in order to prevent one additional bad outcome. It is an important way to present the results of a study or assess the usefulness of treatment or prophylaxis. Sometimes, it is more convenient for practitioners to use NNT than measures of association (which represents the strength of association, not the practical aspects of treatment efficacy). The calculation of NNT is easy: it is actually the inverse of absolute risk reduction (ARR). ARR = Control group event rate - Experimental group event rate = 8/1000 - 6/1000 = 2/1000 - - - - NNT = 1/ARR = 1/0.002 = 500 Therefore, according to the study results, 500 males should be circumcised in order to prevent one case of penile carcinoma. Educational Objective: NNT is the number of patients who need to be treated in order to prevent one additional bad outcome; it is actually the inverse of absolute risk reduction.

A Follow-Up Study Was Conducted to Assess the Effect of Circumcision