Direct standardisation ppt

Standardization in epidemiological research

1. Direct standardisation PADDINGTON MUNDAGOWA

2. standardisation is a set of techniques used to remove as far as possible the effects of differences in age or other confounding variables when comparing two or more variables The methods are developed to distinguish: Target variable Population to be compared & a standard population Variables by which we standardize confounding factors A target function or target parameter Comparison of crude rates may not be sensible

3. Factors influencing population’s mortality Gender Place of residence Economic activity Age Education Family status

4. Direct standardisation Choice of standard population: considerations When several different populations are being compared, a ‘pooled’ standard minimizes the variance of the adjusted rates In examining trends, an appropriate standard is one that reflects the average structure of the population over the time period The standard should be similar to the population of interest It should not change frequently (all historic data would need to be recomputed) It should be used consistently to ensure comparability of rates Choi , 1999. Am J Epi

5. Target population(s ) – the population(s) that we are interested in. Standard population – the population that we use to construct comparisons with and between target populations .

6. E.g. comparing deaths in zimbabwean towns gweru mutare Age /years Pop size deaths Pop size deaths <45 328 049 401 89 893 114 45-64 142 670 1 016 14 947 90 65+ 92 168 3 605 2 077 81 TOTAL 562 887 5022 106 917 285

7. Crude death rate for Gweru or Mutare =  (deaths for all age groups)  ( pop S for all age groups) Age-specific death rate is the crude death rate for a specific age group Gweru crude death rate = 5022/562,887 = 8.9 per thousand Mutare crude death rate = 285/106,917 = 2.7 per thousand Is Gweru really that much worse?

8. Directly standardized rates Relative sizes of age groups affect crude rate comparison. Weighting of age specific rates differs between target populations. Use standard age group sizes and apply age specific rates to these. Result is a directly standardised rate.

9. Zim Gweru Mutare Age/years Pop% Pop% Pop% <45 67 58 84 45-64 22 25 14 65+ 11 17 2 Total 100 100 100

10. Directly standardised rate Directly age standardised rate for Gweru = Σ(standard weight x age specific rate) Σ(standard weights) where sum (Σ) is over all age groups, and weight = size of standard population for each age group

11. Age/years Zim pop Age specific rate Gweru * death rate per thousand population Age specific rate Mutare <45 67 1.2 1.3 45-64 22 7.1 6.0 65+ 11 39.1 39.0 Total 100 6.7 6.5 Crude rates 8.9 2.7

12. Direct standardisation Gweru : {(67x1.2)+(22x7.1)+(11x39.1)}/ 100= 6.9 Direct standardisation applies age specific rates from the target population(s) to the age group sizes in a standard population. Answers the question: What would the rate in the standard population be if it had the same age specific rates as the target population? Allows comparison between target populations. How do you choose the right standard population? Relevant to target population(s) e.g. DRC population for Hb comparisons Appropriate for comparison being made e.g. hospital population for surgery outcome data

13. conclusion Direct standardisation applies age-specific rates from the target population to the age group structure of a standard population . Target Population= group specific rates Standard population= group population sizes

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