DIRECT STANDARDISATIONPADDINGTON MUNDAGOWA

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 variablePopulation to be compared & a standard populationVariables by which we standardize confounding factorsA target function or target parameter• Comparison of crude rates may not be sensible

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

DIRECT STANDARDISATIONChoice 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

• 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.

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

Crude death rate for Gweru or Mutare =

(deaths for all age groups)(popS 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?

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.

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

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

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

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

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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