- C Carazo-Díaz: Universidad Católica de Murcia, Murcia, España.
- L Prieto-Valiente: Universidad Católica de Murcia, Murcia, España.
One of the most common errors made by physicians in all developed countries is to say that the p-value of a test is the probability that the null hypothesis considered in the test is true or false. Eighty percent of those polled in many surveys make this mistake. The p-value of a test is the probability of obtaining a result like the one obtained in the investigation if the null hypothesis is true. The probability of a pregnancy involving three embryos is very small, at 0.00008 (8 in 100,000). In pregnancies with triplets, the probability of a caesarean section being performed is very high, at 0.98 (98%). These are two very different values, and two very different concepts. Saying that 0.98 is the probability of a pregnancy involving triplets would be a serious mistake. We make the same mistake when we say that the p-value of the test is the probability that the null hypothesis is true, or the probability that it is false.