On establishing reference values.

In order to establish a range of reference values for any characteristic one can use Gaussian or nonparametric techniques, whichever are most appropriate. One has the choice of calculating tolerance intervals or percentile intervals. A tolerance interval is said to contain, say 95% of the population with probability, say 0.90. A percentile interval simply simply calculates the values between which 95% of the observations fall. If the data can be said to have a Gaussian distribution, the same precision can be obtained with smaller sample sizes than using the nonparametric techniques. In some cases, data which are not Gaussian can be transformed into a Gaussian form and hence make use of the more efficient Gaussian techniques. In both cases, the data should be checked for outliers or rogue observations and these should be eliminated if the testing procedure fails to imply that they are an integral part of the data.