Organ or tissue weighting factors
The effective dose is calculated by multiplying the 'equivalent dose' absorbed by each organ with a weighting factor specific to it and then adding the total. The sum of all the different weighting factors must be equal to 1. As time goes on and data about our bodies sensitivity become more accurate, the values of these weighting factors have changed. The values put in place in 1977 (in blue), for instance, are less accurate than those updated in 2002 (in red). For this reason effective dose values calculated years ago are not quite comparable with more recent ones.