We have used an age-period-cohort model to calculate these projections. This approach assumes that the probability that someone will get cancer, or die of cancer will depend upon their age, the year that they are born in (period), and which cohort they are in (which depends upon the period).
For example, a 55-year-old today has a different chance of getting mesothelioma than a 55 year old 30 years ago, as there was much greater exposure to asbestos in the work environment 30 years ago than there is now.
We fit this model to historical cancer incidence and mortality rates, with parameters for each of these age, period, and cohort effects. The model fitted to the data calculates a trend which is then used to extrapolate the data further into the future. The impact of this trend is reduced over time, as we don’t want to assume that the same trends will continue forever.
To get the number of cancer cases or deaths from the projected rates, we calculate this as a proportion of the projected population.
Risk factors have been modelled implicitly in this analysis. This means that rather than directly adjusting for, say, smoking rates changing over time, our approach uses the trends seen in the rates of cases and deaths (which are affected by trends in risk factors) to make its projections.
The same is true for the effects of new and improved treatments over time on mortality rates, and other variables such as changes to early diagnosis. The effects of screening was also implicitly modelled for bowel and cervix cancers, and explicitly modelled for breast and prostate cancers.
It is not possible to assess the statistical significance of changes between 2014 (observed) and 2035 (projected) figures. Confidence intervals are not calculated for the projected figures. Projections are by their nature uncertain because unexpected events in future could change the trend. It is not sensible to calculate a boundary of uncertainty around these already uncertain point estimates. Changes are described as ‘increase’ or ‘decrease’ if there is any difference between the point estimates.
For full details of the methods used in these projections, see Smittenaar et al (2016).
- Smittenaar CR, Petersen KA, Stewart K, Moitt N. Cancer Incidence and Mortality Projections in the UK Until 2035. Brit J Cancer 2016.