Comparison of different
populations: standardization, direct
and indirect standardisation, SMR.
Factors influencing a population’s mortality
Age
Gender
Place of residence
Population’s mortality
Education
Family status
Economic activity
Standardization
• „…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.”
Methods of standardization
• There are two methods of standardisation commonly used in
epidemiological studies, and these are characterized by whether the
standard used is a population distribution (direct method) or a set of
specific rates (indirect method). Both direct and indirect
standardisation involves the calculation of numbers of expected
events (e.g. deaths), which are compared to the number of observed
events.
Why standardize?
Examining crude rates alone can be misleading if underlying populations
are different (age-specific rates are better)
But
Cumbersome to compare age-specific rates especially when doing large
number of comparisons
Slide from: Nam Bains
Concept of direct
standardization
Example 1
Step 1 – Calculate age-specific mortality
• Calculate the age-specific mortality rates for each age group in each
population
How many age categories?
Lots (detailed age groups)
• better control of the effect of any differences in age distributions but,
• lots of strata means there might not be enough events (larger variance)
Fewer (broad groups)
• will produce less precise adjustment
• broad groups (i.e., 65+) will not be sensitive to changes in age-specific rates within
that group
Other considerations
• availability of data (i.e., CCHS)
Slide from: Nam Bains
Step 1 - Calculation
Step 1 - Results
Step 2 – Choose a standard population
Standard populations are "artificial populations" with fictitious age structures, that are used in age standardization
as uniform basis for the calculation of comparable measures for the respective reference population(s).
12,000
10,000
USA 1940
Canada 1991
World “Segi”
USA 2000
European
WHO World
8,00 0
6,00 0
4,00 0
2,00 0
12,000
10,000
8,00 0
6,00 0
4,00 0
2,00 0
-
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
Step 2 – Choose a standard population
Step 3 – Project Swedish and Mexican
mortality rates onto the standard population
Step 3 - Calculate
Mexico
Sweden
Expected
Mortalit Expected
St. Pop Mortality deaths St. Pop.
y
deaths
0-29
51000
1,7‰
86,7
51000
0,4‰
20,4
30-59
37000
4,1‰
151,7
37000
2,4‰
88,8
60+
12000
39,6‰
475,2
713,6
12000
43,2‰
518,4
627,6
Step 4: Calculate standardized mortality rates
for Mexico and Sweden
• Mexico= 713,6/100 000*1000 = 7,136
• Sweden= 627,6/100 000*1000 = 6,276
Step 4 – Calculate Comparative Mortality
Ratio (CMR)
• CMR: Standardized mortality rate of Mexico/Standardized mortality
rate of Sweden
• CMR=1,13
• Mortality in Mexico is 13% higher than in Sweden.
Concept of indirect
standardization
When do we use indirect standardization?
• The indirect method of standardisation is commonly used when agespecific rates are unavailable. For example if we did not know the age
specific mortality rates for country B (see next slide).
• In this method, instead of taking one population structure as standard
and applying sets of rates to it to estimate expected events, a set of
rates from a standard population (country A, see next slide) is applied
to each of the populations being compared to calculate standardized
morbidity/mortality ratios.
• Indirect standardization may be also used if we want to compare a
small population (e.g. province, city, workers in a given factory) to a
larger population (e.g. country). See Example 3.
When do we use indirect standardization?
• The indirect method of standardisation is commonly used when agespecific rates are unavailable. For example if we did not know the age
specific mortality rates for country B (see next slide).
• In this method, instead of taking one population structure as standard
and applying sets of rates to it to estimate expected events, a set of
rates from a standard population (country A, see next slide) is applied
to each of the populations being compared to calculate standardized
morbidity/mortality ratios.
• Indirect standardization may be also used if we want to compare a
small population (e.g. province, city, workers in a given factory) to a
larger population (e.g. country). See Example 3.
Example 2
• In Country B 15 300 people died. We would like to compare the
number of deaths with Country a. Unfortunately we don’t know the
number of deaths in Country B. What do we do?
Step 1 - Calculate how many deaths would be
expected in Country B if it had the same agespecific mortality rates as Country A
Step 2 – Calculate standardized mortality ratio
(SMR)
• SMR = Observed number of deaths (O) X 100
Expected number of deaths (E)
• Mortality in country B is 60% higher than the number we would
expect if Country B had the same mortality experience as Country A.
Example 3
• Calculate how the number of deaths in a fictitious chemical factory in
Mexico relates to the number of deaths in the whole Mexican
population.
Practice
20 years after the American National Health And Nutrition Examination Survey –
NHANES, 1971-75 Gu et al was trying to find out if there was a difference in mortality
between 1971 and 1993 of those claimed themselves diabetic in 1971 compared to the healthy
population. The following table shows some of their results:
Male
Diabetic
Population Number of death
25-44 years 454
10
45-64 years 1222
60
65-74 years 1484
157
Non-diabetic
Population Number of death
34461
154
28412
706
18189
1371
The standard population of 1990:
Age-group
25-44 years
45-64 years
65-74 years
Population
325,000
186,000
73,000
1. Calculate the standardized mortality of the diabetic population (per thousand)!
2. Calculate the relative mortality risk of the diabetic compared to the non-diabetic
population!
A study examined the prevalence of diabetes in two villages (A and B). The result is shown
by the table:
Age group
15-39
40-59
60+
Total
A village
Population
No. of diabetic
4200
42
3000
450
1200
300
8400
792
B village
Population
No. of diabetic
500
20
600
240
900
540
2000
800
Calculate the prevalence of diabetes in both villages!
Prevalence A:
Prevalence B:
Standardize the data using the following standard population and calculate prevalence again.
Age gorup
15-39
40-59
60+
Standardized prevalence A:
Standardized prevalence B:
Population
6500
5500
3000
A study examined if visiting disco regularly can be an exposition factor for drug-usage. The
result is shown by the table:
Age group
15-20
21-25
26-30
31-35
Total
Non-visiting population
Population
Have ever tried a
drug
25000
525
35000
1190
10000
300
10000
200
80000
2215
Regular disco visitors
Population
Have ever tried a
drug
7750
1248
12250
2217
2000
216
2000
200
24000
3881
Calculate the prevalence of drug usage in both population!
Prevalence among those not attending disco:
Prevalence among those visiting a disco regularly:
Standardize the data using the following standard population and calculate prevalence again.
Age group
15-20
21-25
26-30
31-35
Population
71000
76000
86000
88000
Standardized prevalence among those not attending disco:
Standardized prevalence among those visiting a disco regularly:
The following table presents the mortality rate of two villages (A and B).
Age group
18-35
36-65
66+
Total
A village
Population
No. of death
20000
40
40000
300
24000
1200
84000
1540
B village
Age group
No. of death
12000
36
30000
300
20000
800
62000
1136
Calculate the crude mortality in both villages!
Mortality A:
Mortality B:
Standardize the data using the following standard population and calculate mortality again.
Age group
18-35
36-65
66+
Standardized Mortality A:
Standardized Mortality B:
Population
65000
55000
30000
Example: SMR for Male Farmers, England and
Wales, 1951
Number of Farmers and
Farm Managers
Age
Group
20-24
25-34
35-44
45-54
55-64
(Census, 1951)
(1)
7,989
37,030
60,838
68,687
55,565
Standard Death Rates per
1,000,000 (All Causes of Death)
(2)
1,383
1,594
2,868
8,212
22,953
Total observed deaths per year among farmers: 1,464
SMR = ?