Population statistics cannot be used for reliable individual prediction

It is known that predictions about individuals from statistical data about the population are in general unreliable. However, the size of the problem is not always realised. For a number of ways of predicting information about one variable from another with which it is correlated, we compute the reliability of such predictions. For the bivariate normal distribution, we demonstrate that unless the correlation is at least 0.99, not even the sign of a variable can be predicted with 95% reliability in an individual case. The other prediction methods we consider do no better. We do not expect our results to be substantially different for other distributions or statistical analyses. Correlations as high as 0.99 are almost unheard of in areas where correlations are routinely calculated. Where reliable prediction of one variable from another is required, measurement of correlations is irrelevant, except to show when it cannot be done.