ISO 16269-6:2013 describes procedures for establishing statistical tolerance intervals that include at least a specified proportion of the population with a specified confidence level. Both one-sided and two-sided statistical tolerance intervals are provided, a one‑sided interval having either an upper or a lower limit while a two-sided interval has both upper and lower limits. Two methods are provided, a parametric method for the case where the characteristic being studied has a normal distribution and a distribution‑free method for the case where nothing is known about the distribution except that it is continuous. There is also a procedure for the establishment of two‑sided statistical tolerance intervals for more than one normal sample with common unknown variance.