Choice of standard population in direct standardisation

1. Choice of standard population in direct standardisation Hugo CoshHealth Information Analysis Team (HIAT)

2. Background • HIAT routinely uses direct standardisation to facilitate comparison between rates of events in populations with different age structures • The European standard population, created approx 1970, is used as the weighting for standardisation

4. Age < 1 Age 1-4 Age 85+ Standard pop Study pop Standard pop Study pop Standard pop Study pop x x x events events events Directly standardised rate (DSR) calculation: weighted average + … + + Sum of standard population Standard pop Study pop is important – it governs weighting

5. Weighting Standard pop Study pop

6. Weighting European Wales Events heavily weighted Events lightly weighted

7. Worked exampleCoronary heart disease (CHD) mortality, 2003-05, Wales Local Health Boards

8. CHD mortality, age-specific rate per 100,000, Wales LHBs, 2003-05 Source: ONS

9. European standard Weighting Weighting Wales LHB (2003-5) European LHB (2003-5) Wales standard

10. x Standard pop LHB pop events Wales standard European standard % of total ASR

11. Lower Significantly Lower Significantly Higher Higher DSR compared with Wales Wales standard European standard

13. Conclusions • Choice of standard population impacts on results when directly standardising rates • European standard population systematically places higher weighting on events in younger age groups and lower weighting on events in older age groups • Wales standard population provides a more even system of weighting, reducing bias in the calculation

14. “It was found that the choice of population standard affects the magnitude of mortality rates, rate ratios and rate differences, the relative ranking of causes of death, and the relative width of confidence intervals. This in turn will affect the monitoring of trends in health outcomes and health policy decision-making.” Robson B, Purdie G, Cram F, Simmonds S (2007) Age standardisation – an indigenous standard?Emerging Themes in Epidemiology 4(3)

15. “Always choose a standard population similar to the study populations. For example, if the units of interest are all the local authorities in England, then use the national population; if the units of interest are two wards in a local authority, then the local authority population or the average of the two wards may be appropriate.” ERPHO (2006)

16. “Age-adjustment is an averaging process, and consequently, has the potential to view the data effectively as a whole while possibly obscuring important age-specific details.” Sorlie PD, Thom TJ, Manolio T, Rosenberg HM, Anderson RN, Burke GL. (1999) Ann Epidemiol. 1999 Feb;9(2):93-100

17. Geographical level sub-Wales, e.g. LHB or small area All-Wales European standard Continuity required with previous analyses? Small area cluster analysis? LHB / LA standard Wales standard Recommendations (1): choice of standard population

18. Recommendations (2): premature mortality • European standard population systematically places higher weighting on events in younger age groups – may be desirable for public health analyses. • Along Wales age-standardardised rates, alternative methods of analysing premature mortality to be investigated e.g. potential years life lost, probability of survival

19. Recommendations (3): age-specific rates • Difficulty in presenting volume of information in user-friendly format – investigate graphical methods of presentation, e.g. age-specific rate pyramids