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ORIGINAL ARTICLEMalaysian Journal of Medical Sciences, Vol. 7, No. 1, January 2000 (10-15)

EASY WAY TO LEARN STANDARDIZATION :DIRECT AND INDIRECT METHODS

Nyi Nyi Naing

Unit of Biostatistics & Research MethodologySchool of Medical Sciences, Universiti Sains Malaysia

16150 Kubang Kerian, Kelantan, Malaysia

In direct age-adjustment, a common age-structured population is used as standard.This population may actually exist (e.g., United States population, 1999) or maybe fictitious (e.g., two populations may be combined to create a standard). In indirectage-adjustment, a common set of age-specific rates is applied to the populationswhose rates are to be standardized. The simplest and most useful form of indirectadjustment is the standardized mortality ratio (SMR) (5).

Key words : easy way, standardization methods, direct, indirect

Introduction

Comparing mortality and morbidity rates intwo or more different geographic areas is importantfor the evaluation of community health status. Asthere is a possibility of having different frequencydistributions in different populations, a comparisonbetween crude rates would be misleading since cruderates are not very informative about the health statusof a population. Standardization for thecharacteristic(s) responsible for the differences incomparison is necessary. Age and sex are two of themost common variables used for standardization andthey are called standardized rates. The differencebetween crude rates and standardized rates is thatcrude rates are calculated based on the populationunder study as a whole whereas standardized ratesare based on particular characteristic(s) as standard(Figure 1). If the rates are calculated based on thespecific characteristic(s), they are called specificrates (e.g. age specific mortality rate).

This article attempts to help health personnelin the selection and utilization of appropriatestandardization methods using illustratedexplanations. There are two methods for calculatingstandardized rates, namely direct and indirect

standardization. For the example purpose, let usconcentrate on the standardization methods basedon age-standardized rates.

When age-specific mortality rates for two ormore populations are known, direct standardizationmethod can be applied.

Procedure for direct standardization

Calculate the age-specific mortality rates foreach age group in each population. Then choose thestandard (reference) population from one of thepopulations (*Note: If the mortality rates of aspecific community are compared to the nationalpopulation, then the national population isconsidered as a “standard” population). Multiply theage-specific mortality rates of the other populationunder study to the number of persons in each agegroup of the standard population. By this way, youwill get the expected deaths for each age group ofeach population. Add the number of expected deathsfrom all age groups. Finally to get the age-adjustedmortality rates, divide the total number of expecteddeaths by the standard population (1-4). Now youcan conclude by comparing the age-standardizedmortality rates of two populations (figure 2).

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EASY WAY TO LEARN STANDARDIZATION : DIRECT AND INDIRECT METHODS

Figure 1: Concept of direct standardization

Population (s)of

Interest

Age-specificMortality

Rates(Known)

StandardPopulation

Compare theAge-adjusted

MortalityRates

Figure 2: Procedure for application of direct standardization method

Age Population I No. of deaths Agegroups in each age specific

group death rate

Age Population II No. of deaths Agegroups in each age specific

group death rate

B1 A1

CDR1=A1/B1

B2 A2

CDR2=A2/B2

Age Standard Expected Agegroups Population deaths specific

death rate

B1 A1

ADR1=A1/B1

B2 A2

ADR2=A2/B2

Age Standard Expected Agegroups Population deaths specific

death rate

*

CDR = Crude Death Rate ADR = Adjusted Death Rate

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Summation of the total number of expected deaths

Population A = 1186.18

Population B = 1158.99

Age adjusted death rate for population A= 1186.18 x 1000 51,000= 23.3 per 1,000 population

Age adjusted death rate for population B= 1158.99 x 1000 51,000= 22.7 per 1,000 population

Commenting on age-adjusted rates, in fact therisk of death is higher in population A than inpopulation B. It has clearly shown that you may havemisleading conclusion if you rely only on crudedeath rates.

Nyi Nyi Naing

Example1

Table 1: Age-groups, deaths and mid-year populations of two different populations

Commenting on crude death rates, population B seems to have higher death rates than population A

As an example, let us say the national population has been chosen as the reference population, thecalculation will therefore be as follows in table 2

Table 2: Calculation of expected deaths by applying direct standardization method

Age group Reference Age-specific Expected Age-specific Expected(years) population death rate deaths death rate deaths

per 1000 per 1000

0-2425-4950-7475 and above

Total

11,00017,00020,0003,000

51,000

21.3492.65822.20249.99

1186.18

1.945.4541.1183.33

2.317.1436.3695.00

25.41121.38727.20285.00

1158.99

Age group Mid-year Deaths Age-specific Mid-year Deaths Age-specific(years) population death rate population death rate

per 1000 per 1000

0-2425-4950-7475 and above

Total

Crude rate per1000

18,00011,0009,0003,000

41,000

3560370250

715

1.945.4541.1183.33

17.44

13,0007,00011,0004,000

35,000

3050400380

860

2.317.1436.3695.00

24.57

Population A Population B

Population A Population B

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Age Population II No. of deaths Agegroups in each age specific

group death rate

When Age-specific mortality rates of the population (s) of interest are unknown, indirect standardizationmethod is applied

Figure 3: Concept of indirect standardizationAge-specific

MortalityRates

(unknown)

Compare theadjusted

mortality Rates

Population (s)of

Interest

ReferencePopulation

Compare theStandardized

mortality Ratiosof the

population(s) ofinterest

Age Population II Expected Agegroups deaths specific

death rate

Figure4: Procedure for application of indirect standardization methodAge Population I No. of deaths Agegroups in each age specific

group death rate

B1 Obs1

CDR1=Obs1/B1

B2 Obs2

CDR2=Obs2/B2

Age Population I Expected Standard Agegroups deaths specific

death rate

Exp1

SMR1=Obs1/Exp1

Exp2

SMR2=Obs2/Exp2

*

EASY WAY TO LEARN STANDARDIZATION : DIRECT AND INDIRECT METHODS

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Let us see the second method which is anindirect standardization.

Procedure for indirect standardization

Choose a reference or standard population.Calculate the observed number of deaths in thepopulation (s) of interest. Apply the age-specificmortality rates from the chosen reference populationto the population(s) of interest. Multiply the numberof people in each age group of the population(s) ofinterest by the age-specific mortality rate in thecomparable age group of the reference population.Sum the total number of expected deaths for eachpopulation of interest. Divide the total number ofobserved deaths of the population(s) of interest bythe expected deaths (figure 4) (1-4).

The ratio of the observed number of deaths

to the expected number of deaths is called:“Standardized mortality ratio” or SMR

SMR = Observed number of deaths Expected number of deaths

Adjusted mortality rates (AMR) can becalculated by the following formula:-

Adjusted mortality rates = Standardizedmortality ratio x crude death rate

AMR= SMR x CDR (Standard)

(* Note : if the age-specific mortality ratesof the reference population is applied, Crude DeathRate must be calculated from that referencepopulation.)

Age Population Age-specific Expected Population Age-specific Expectedgroup mortality rate deaths mortality rate deaths

per1000 per1000

0-2425-4950-7475+

Total

2000250035004500

4.07.010.030.0

8.017.535.0135.0

195.5

1000150025001000

4.07.010.030.0

4.010.525.030.0

69.5

Example (2)

Let us say the observed deaths in the populations A and B are as follows:

Observed deaths in population A=120 Observed deaths in population B= 30

Table 3: Calculation of expected deaths by applying indirect standardization method

Division of the total number of observed deaths by the total number of expected deaths

SMR for population A = 120 = 0.61195.5

SMR for population B = 30 = 0.4369.5

The risk of death is in fact higher in population A than population B after adjusting for differences by age.Common practice is to compare (SMR) in indirect method.

Population A Population B

Nyi Nyi Naing

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Conclusion

Standardization methods are not difficult butsometimes the health personnel have some confusionabout selecting which method and how to calculateand apply the particular method. It is sincerely hopedthat this article may at least contribute to publichealth medicine by improving the understanding ofstandardization methods in comparing two or moredifferent populations, which have difference(s) insome characteristic(s).

Correspondence :

Nyi Nyi NaingUnit of Biostatistics & Research MethodologySchool of Medical SciencesUniversiti Sains Malaysia16150 Kubang Kerian, Kelantan, Malaysia

References

1. David J. Hall, Standardization and life tables. IslamicWorld Medical Journal. 1984 ; 14(4) :22-25.

2. Christie D, Gordon I and Heller R. Epidemiology.Mortality and Morbidity: Comparisons of time andplace. 1994 ; 4 : 26-36.

3. Health Indicators. Epidemiology A, Basic MethodsModule.Center for Clinical Epidemiology andBiostatisitcs. University of Newcastle, Australia, 1998:12-20.

4. Supanvanich S., Podhipak A. Principles ofEpidemiology. 1993 :103-109

5. Mausner J.S., Bahn A.K. Epidemiology an introductorytext. 1974 ; 7 : 146-159.

EASY WAY TO LEARN STANDARDIZATION : DIRECT AND INDIRECT METHODS