With the year-to-year percentage change in smoking rate forecasts from 2010 forward, we change the year-to-year values of the smoking impact series 25 years later. Thus, we use the reverse process used earlier–rather than using changes in smoking impact to compute smoking rates, we use changes in smoking rates to compute smoking impact.
One complication is that we need smoking impact by age-category. We first compute smoking impact in the 4 big age categories of the GBD data. For purposes of affecting mortality we then apply the large age category values to each of the underlying 5-year categories; for example we compute smoking impact for 30-44 year olds, and then we apply the value for 30-34, 35-39, 40-44 year olds.
Even in the big categories, however, as well as in the smaller 5-year categories, a consequence of our approach is that the changes in smoking impact year-to-year are identical in each age category. This is an inevitable consequence of our not having smoking rates by age–we have no basis for positing different patterns of smoking over time by age and therefore for changing the smoking impact by age.
An unfortunate corollary consequence of this procedure is that, in the years between our initial forecast year and 2030, a period for which we have the smoking impact forecasts of the GBD but are also computing our own smoking impact forecasts, our forecasts will differ somewhat from those of the GBD. Remember, however, that we used GBD forecasts to create historical smoking rate series, and in those forecast years through 2030 are computing smoking impact from the historical smoking rates; thus our smoking impact forecasts through 2030 will not differ greatly from those of the GBD. It would be possible for us to actually use the GBD smoking impact forecasts for the 4 age categories through 2030 and then initiate our own changes to all 4 categories; we judged that the computational intensity of using theirs through 2030 offset the advantages of doing so.
Note that it is possible to stop changes in the Smoking Impact forecast in year 2030 and beyond by turning off the smoking impact switch, hlsmimpsw = 0.