Some properties are considered of two-sided 3-content tolerance intervals for a normal distribution. We are given the sample mean, x, and the sample standard deviation, s, computedfrom a random sample of size n from a N(l,az) distribution. The tolerance interval x ± ks will cover a proportion, 3, of the sampled distribution with (100)confidence. We consider the effect of sample size on the proportion of the population covered, using a criteria for measuring the precision of the coverage. We also give tables to enable the user to control this precision. 969