Non-mathematical explanation of the method used. Risk pyramids are shown on the left and households on the right. Top: If cases and non-cases are straightforwardly ascertained, then within-household transmission will tend to cluster the cases so that there are either many or few cases within a household. Bottom: The presence of further stratification complicates the picture if information is only available on one outcome, but if full information is available, the clustering of cases by household is still visible and gives an accurate picture of transmission. Case ascertainment is not explicitly represented in this cartoon, but the principle is similar.