Extrapolation: survey data on the prevalence of slavery (expressed as a percentage of the population) that was available for each cluster was then used to estimate the prevalence for other countries within that same cluster. A slightly different method was followed for 'cluster 1', which involved mostly European and highly developed countries. This is discussed further below and in the article by Bales and Datta.[4] National-level adjustments were done by hand on a country-by-country basis, reflecting considerations such as geography.