The poor correlation between decline in lung function (FEV1) and loss of lung density (PD15), the slopes, is probably explained by the relatively short time frames of the 2 studies (4 years for DLCST and 3 years for ECLIPSE). Within a time frame of 3-4 years the changes in both PD15 and FEV1 are relatively small as compared to the measuring error which for both parameters is larger than the annual change.37,49 This is illustrated in Figure 2 where baseline values (Figure 2A) and annual changes are plotted on the same scale (Figure 2B). The figure shows how small the annual changes are as compared to the changes that have accumulated at an age of 60 years which was the mean age of the participants.