In estimating risks occasionally very high and very low risks can be generated. It may be that there is a degree of uncertainty as to whether these risks are accurate. For example, if a woman had a risk estimate of having a pregnancy with Down's syndrome of 1 to 100,000 there may be too few empirical data to validate this. In this event it may become appropriate to, say, cap the risk as being less than or equal to 1 in 50,000. Similarly with a high risk of say, 20 to 1 one might wish to limit this to say equal to or greater than 5 to 1.