BOWEN UNIVERSITY ,IWO,OSUN STATE,NIGERIA.STATISTICS PROGRAMME

STANDARDIZATIONSTA323(Demography) Note

A.C OMOYAJOWO

adefunke.omoyajowo@bowen.edu.ng

LEARNING OUTCOMES

At the end of this class, students should be able to:

Understand the concept of standardization

Understand the methods of standardization and their uniqueness

Compute the standardization of countries

Interprete the standardization results

CONCEPT OF STANDARDIZATION

One method of overcoming the effects of confounding variables such as age is to simply present and compare the age-specific rates. While this stratification allows for a more comprehensive comparison of mortality or morbidity rates between two or more populations, as the number of stratum-specific rates being compared increases, the volume of data being examined may become unmanageable.

It is, therefore, more useful to combine category specific rates into a single summary rate that has been adjusted to take into account the population’s age structure or another confounding factor. This is achieved by using the methods of standardization

METHODS OF STANDARDIZATION

There are two methods of standardization commonly used. They are characterized by whether the standard used is either:

A population distribution (direct method)

A set of specific rates (indirect method)

Both direct and indirect standardization involves calculating the number of expected events (e.g. deaths) and then comparing this to the number of observed events.

Age is a factor that is frequently adjusted for in epidemiological investigations, particularly in comparative mortality studies, since the age structure of a population will greatly affect the populations overall mortality.

To illustrate the methods of both direct and indirect standardization the age specific mortality rates for two hypothetical populations are compared below:

Direct standardization

In the direct method of standardization, 'age-adjusted rates' are derived by applying the category-specific mortality rates of each population to a single standard population. This produces age-standardized mortality rates that these countries would have if they had the same age distribution as the standard population.

Note that the 'standard population' used may be the distribution of one of the populations being compared or may be an outside standard population such as the WHO’s World Standard Population.

STEPS INVOLVED IN DIRECT STANDARDIZATION

The steps involved in direct standardization are as follows:

1. Identify a standard population for which relevant stratum-specific data are available

2. Calculate the number of stratum-specific expected deaths, as follows:

For each age stratum of each population being compared, multiply the age-specific mortality rate by the size of the standard population for that stratum. This essentially gives you the number of deaths one would expect in the standard population if it had the same mortality rates as your study population.

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STEPS INVOLVED IN DIRECT STANDARDIZATION cont’d

3. Calculate the total number of expected deaths by summing all the values from the stratum-specific calculations, above. This gives the total number of deaths that would be expected in the standard population if it had the same mortality rate as your study population.

4. Calculate the age-standardized rate by dividing the total number of expected deaths by the total standard population size.

EXAMPLE 1TABLE 1:Crude mortality rates stratified by age for

two hypothetical populations.

Country A Country B

Age -group No.of deaths PopulationMortalityRate per

1,000 pyrsNo. of deaths Population

Mortality Rate per

1,000 pyrs

0-29 7,000 6,000,000 1.2 6,300 1,500,000 4.2

30-59 20,000 5,500,000 3.6 3,000 550,000 5.5

60+ 120,000 2,500,000 48 6,000 120,000 50

Total 147,000 14,000,000 10.5 15,300 2,170,000 7

EXAMPLE 1 cont’d

From Table 1 above, compare the mortality rate of the two countries using Direct Standardization method applied to the standard population below:

Age groupNumber of people in a hypothetical standard population

0-29 100,000

30-59 65,000

60+ 20,000

Total 185,000

SOLUTION

TABLE 2: Direct method of standardization - calculation of the

number of expected deaths for countries A and B applied to a standard population.

Country A Country B

Age stratum Expected deaths Expected deaths

0-29 0.0012 x 100,000 = 120 0.0042 x 100,000 = 420

30-59 0.0036 x 65,000 = 234 0.0055 x 65,000 = 357.5

60+ 0.048 x 20,000 = 960 0.05 x 20,000 = 1,000

Total expected deaths 1,314 1,777.5

Age adjusted rate 1,314/185,000 =7.1 per 1,000 pyrs

1,777.5/185,000 =9.6 per 1,000 pyrs

SOLUTION cont’d

The overall crude mortality rate is higher for country A (10.5 deaths / 1,000 person years) compared with country B (7 deaths / 1,000 person years), despite the age-specific mortality rates being higher among all age groups in country B.

The age-standardized rate provides a single summary measure for each population of interest that reflects the numbers of events that would have been expected if the populations being compared had the same age distribution.

The ratio of the two directly standardized rates can then be calculated to provide a single summary measure that reflects the difference in mortality between the two populations. The ratio of the standardized rates is called the Comparative Mortality Ratio (CMR) and is calculated by dividing the overall age-standardized rate in, say, country B by the rate in country A.

Comparative Mortality Ratio = 9.6/7.1 = 1.35

INTERPRETATION OF RESULT

After controlling for the confounding effects of age, the mortality rate in Country B is 35% higher than in country A (1.35%=135, 135-100=35% higher)

Note: While the values of the age-standardized rate do not reflect the 'true' mortality experience of countries A and B, the direct method of standardization allows one to make a valid comparison of overall mortality between the two countries.

INDIRECT STANDARDIZATION METHOD

The indirect method of standardization is commonly used when age-specific rates are unavailable. For example, if we did not know the age-specific mortality rates for country B, we could not have applied direct standardization.

In indirect standardization, instead of taking one reference population structure as the standard and applying both sets of mortality rates to this to estimate expected events, a known set of stratum-specific rates (from either one of the populations being compared, or from a standard population) is applied to the structure of each of the populations being compared. This calculated expected rate can be compared with the overall observed rates to give a standardized morbidity/mortality ratio (SMR).

STEPS INVOLVED IN INDIRECT STANDARDIZATION

1. Identify a population with stratum-specific death rates: In this case, we will use the rates from country A as the comparator.

2. Calculate the expected numbers of stratum-specific expected deaths: This is done by multiplying each stratum population size by the corresponding mortality rate of the comparator population.

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3. Calculate the total number of expected deaths: By summing the number of expected deaths in each stratum. Note that if one of the study populations is being used as the “standard” comparator population, the number of expected deaths will always equal the number of observed deaths. As such, it doesn’t need to be calculated from scratch.

4. Calculate the SMR : This is the ratio between the observed and expected number of deaths

Note that the SMR is always expressed as a percentage.

EXAMPLE 2

From Table 1 above, using Country A as the standardized population, obtain the indirect standardization method to calculate the SMR

Age group Country A Country B

Expected deaths Expected deaths

0-29 0.0012 x 6,000,000 =7,200 0.0012 x 1,500,000 =1,800

30-59 0.0036 x 5,500,000 =19,800 0.0036 x 550,000 = 1,980

60+ 0.048 x 2,500,000 =120,000 0.048 x 120,000 = 5,760

Total expected deaths (E) 147,000 9,540

Total observed deaths (O) 147,000 15,300

EXAMPLE 2 cont’d

In this example, the expected number of deaths in Country B are calculated by multiplying the age-specific rate for Country A by the population of Country B in the corresponding age group. The sum of the age categories give the total number of deaths that would be expected in country B, if it had the same mortality experience as country A.

The SMR for country B is calculated by

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15300

9540∗ 100 = 160%

Interpretation

Note that by this method, the comparator population (in this case country A) has, by definition, an SMR of 100. The number of observed deaths in Country B is therefore 60% higher than what we would expect if Country B had the same mortality experience as Country A.

SUMMARY OF THE USE OF STANDARDIZATION

Standardized rates are used for the comparison of two or more populations; they represent a weighted average of the age-specific rates taken from a 'standard population' and are not actual rates.

The direct method of standardization requires that the age-specific rates for all populations being studied are available and that a standard population is defined.

The indirect method of standardization requires the total number of cases (e.g. number of deaths).

The ratio of two directly standardized rates is called the Comparative Incidence Ratio or Comparative Mortality Ratio.

SUMMARY OF THE USE OF STANDARDIZATION cont’d

The ratio of two indirectly standardized rates is called the Standardized Incidence Ratio or the Standardized Mortality Ratio.

Indirect standardization is more appropriate for use in studies with small numbers or when the rates are unstable.

As the choice of a standard population will affect the comparison between populations, it should always be stated clearly which standard population has been applied.

Standardization may be used to adjust for the effects of a variety of confounding factors including age, sex, race or socio-economic status.

References

Carneiro I, Howard N. Introduction to Epidemiology. Open University Press, 2011.

Hennekens CH, Buring JE. Epidemiology in Medicine, Lippincott Williams & Wilkins, 1987.

THANK YOU FOR READING!