Standardization of rates

STANDARDIZATION OF RATES

Halyna Lugova, MD, PhD

October 8, 2014

Standardization of Rates: Town B: affluent rural

community, popular retirement area

The all-cause crude death rate – 14.2 per 1,000/year in 2013

2

Town A: high unemployment rates, poverty

The all-cause crude death rate – 11.1 per 1,000/year in 2013

This suggests that mortality is higher in Town B

although this is not what we would expect given

socioeconomic characteristics of the two towns

Standardization of Rates: Age distribution for two populations

3

0

5

10

15

20

25

30

35

40

45

0-4 5-14 15-24 25-34 35-44 45-54 55-64 65-74 75-84 85 andover

Town A

Town B

X 1

,00

0 p

op

ula

tio

n

age

Standardization of Rates

4

Town A has a younger population, therefore it has a lower

death rate.

How to compare the two towns, independently of the

effects of this difference in age distribution?

We need to have a summary measure of mortality for all

age groups to avoid many tables of rates for each age

group.

Such summary measure that takes account of the

differences in age distribution of the two areas could be

derived by a technique called standardization.

Standardization of Rates 1. The indirect method provides standardized mortality

ratio (SMR) and indirectly standardized rates

2. The direct method provides directly standardized

rates

3. For indirect method we need to select standard

population, e.g. country population, or one of the

‘standard populations’ (not real) created to represent

population structure: World standard population,

European standard population, etc.

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Indirect Standardization – First, we will calculate the SMR for Town A

1. We need to know total number of deaths (‘observed’) in

Town A in 2013

2. We need to know the population in Town A in each age

group in 2013

Indirect Standardization

Age group Deaths

‘observed’: Town A

Population: Town A

Country death rate per 1000/year

(standard population)

Deaths ‘expected’: Town A

0-4 12400

5-14 26900

15-24 42000

25-34 32900

35-44 31700

45-54 27200

55-64 21600

65-74 18400

75-84 11300

85+ 3200

Total 2520 227600

Indirect Standardization 1. We need to know ‘observed’ deaths in Town A


group

3. We choose age-specific deaths rates for a ‘standard’

population; in this case hypothetical country

population


Age group

Deaths ‘observed’:

Town A

Population: Town A

Country death rate per 1000/year (standard

population)

Deaths ‘expected’: Town

A

0-4 12400 1.50

5-14 26900 0.03

15-24 42000 0.32

25-34 32900 0.64

35-44 31700 2.34

45-54 27200 4.02

55-64 21600 6.69

65-74 18400 14.32

75-84 11300 78.30

85+ 3200 180.20

Total 2520 227600

Indirect Standardization 1. We need to know ‘observed’ deaths in Town A


group


population

4. We calculate the numbers of deaths that would have

occurred in Town A – expected deaths, in each age

group, if the ‘standard’ population death rates had

applied.


• For that, we need to multiply the country rate

(column 4) by the Town A population (column 3) in

the same age group

For example, for the 0-4 age group:

𝟏.𝟓𝒙𝟏𝟐𝟒𝟎𝟎

𝟏𝟎𝟎𝟎 = 18.6

• Finally, add up all the age-specific expected deaths to

obtain total number of expected deaths


Age group Deaths

‘observed’: Town A

Population: Town A

Country death rate per 1000/year

(standard population)

Deaths ‘expected’: Town

A

0-4 12400 1.50 18.60

5-14 26900 0.03 0.81

15-24 42000 0.32 13.44

25-34 32900 0.64 21.06

35-44 31700 2.34 74.18

45-54 27200 4.02 109.34

55-64 21600 6.69 144.50

65-74 18400 14.32 263.49

75-84 11300 78.30 884.79

85+ 3200 180.20 576.64

Total 2520 227600 2106.85


1. We need to know ‘observed’ deaths in Town A


group


population

4. We calculate the numbers of expected deaths in town A in

each age group.

5. We can calculate the SMR now


An SMR 120 means that, independently of the influence

of the age distribution in Town A, the overall mortality in

Town A is 20 per cent higher than country average (our

‘standard’ population).

SMR = 𝑻𝒐𝒕𝒂𝒍 𝒏𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 𝒐𝒃𝒔𝒆𝒓𝒗𝒆𝒅 𝒅𝒆𝒂𝒕𝒉𝒔

𝑻𝒐𝒕𝒂𝒍 𝒏𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 𝒆𝒙𝒑𝒆𝒄𝒕𝒆𝒅 𝒅𝒆𝒂𝒕𝒉𝒔 𝒙 𝟏𝟎𝟎

SMR = 𝟐𝟓𝟐𝟎

𝟐𝟏𝟎𝟔.𝟖𝟓 𝒙 𝟏𝟎𝟎 = 𝟏𝟏𝟗. 𝟔 ~ 𝟏𝟐𝟎


– Interpretation of SMR

Independently of the influence of the age distribution, an

SMR

1. of 100% means no difference between the overall

mortality in the population of interest and in the

standard population.

2. >100% means that the overall mortality in the population

of interest is higher than in the standard population.

3. < 100% means that the overall mortality in the

population of interest is lower than in the standard

population.

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Indirect Standardization • To calculate SMR we need to know:

• Age-specific index population data

• Total number of deaths in the index population

• Age-specific deaths rates of the standard

population

• SMR can be calculated if the numbers of

deaths in each age group are not available

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Indirect Standardization Now, we will calculate the SMR for Town B

Age group Deaths

‘observed’: Town B

Population: Town B

Country death rate per 1000/year (standard

population)

Deaths ‘expected’:

Town B

0-4 5200 1.50 7.80

5-14 15100 0.03 0.45

15-24 11300 0.32 3.62

25-34 11100 0.64 7.10

35-44 16600 2.34 38.84

45-54 15400 4.02 61.91

55-64 14900 6.69 99.68

65-74 12700 14.32 181.86

75-84 8800 78.30 689.04

85+ 3100 180.20 558.62

Total 1626 114200 1648.93

Indirect Standardization –We will calculate the SMR for Town B

An SMR 99 means that, independently of the influence of

the age distribution in Town B, the overall mortality in

Town B does not differ significantly (very close to 100)

from that for the ‘standard’ population.

SMR = 𝑻𝒐𝒕𝒂𝒍 𝒏𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 𝒐𝒃𝒔𝒆𝒓𝒗𝒆𝒅 𝒅𝒆𝒂𝒕𝒉𝒔

𝑻𝒐𝒕𝒂𝒍 𝒏𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 𝒆𝒙𝒑𝒆𝒄𝒕𝒆𝒅 𝒅𝒆𝒂𝒕𝒉𝒔 𝒙 𝟏𝟎𝟎

SMR = 𝟏𝟔𝟐𝟔

𝟏𝟔𝟒𝟖.𝟗𝟑 𝒙 𝟏𝟎𝟎 = 𝟗𝟖. 𝟔 ~ 𝟗𝟗

Indirect Standardization –Comparison of SMRs

We cannot compare SMRs between two populations,

e.g. Town A and Town B - only to the standard

population – because the age-specific rate have been

applied to two different populations

What we can state, is that mortality in Town A is 20 per

cent higher than the country average. For Town B,

mortality does not differ significantly from the country

level (very close to 100).

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Direct Standardization

–Direct standardization allows direct comparison

between two populations

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Direct Standardization – We will use population of Town A as the standard and

standardize population of Town B against it

– We will first look at crude death rate for people aged 65 and

over

Town A (standard)

No. of population

Deaths observed

Age specific rate/1000 per year

65-74 18400

75-84 11300

85+ 320

30020 297 9.89

Town B

No. of population

Deaths observed


65-74 12700 45

75-84 880 93

85+ 310 220

13890 358 25.77

Direct Standardization – Now, we will calculate age-specific rates for Town B

Town A (standard)

No. of population

Deaths observed


65-74 18400

75-84 11300

85+ 320

30020 297 9.89

Town B

No. of population

Deaths observed


65-74 12700 45 3.54

75-84 880 93 105.68

85+ 310 220 709.68

13890 358 25.77

Direct Standardization – Standardize the Town B rate to the Town A:

1. Apply age-specific rates of Town B to the age-specific

groups of Town A to calculate the expected numbers in

each group

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Age-specific rate for Town B

Town A (standard) population

Expected deaths for Town B

65-74 3.54 18400 65.20

75-84 105.68 11300 1194.20

85+ 709.68 320 227.10

30020


– Standardize the Town B rate to the Town A:

2. Add up the expected deaths to obtain the total

3. Divide the total expected cases by the total Town A (standard ) population to obtain the age-standardized death rate (x 1,000)

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Age-adjusted rate for Town B

Town A (standard) population

Expected deaths for Town B

65-74 3.54 18400 65.20

75-84 105.68 11300 1194.20

85+ 709.68 320 227.10

49.52 30020 1486.50


– Interpretation

• Before standardization:

– Crude death rate in Town A for people aged 65 and over was 9.89 per 1000 per year

– Crude death rate in Town B for people aged 65 and over was 25.87 per 1000 per year (2.5 times higher than in Town A)

• After standardization of the population of Town B crude death rate to the population of Town A:

– Age-adjusted death rate in Town B for people aged 65 and over was 49.52 per 1000 per year (5 times higher than in Town A)

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– To calculate age-adjusted deaths rates we need

to know:

• Age-specific index population data

• Number of deaths in each age group in index

population

• Age-specific standard population data

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SUMMARY Standardization is applicable for factors other

than age (socio-economic status, race, area of

residence)

Any rates can be standardized, e.g. incidence

Standardization is required to adjust rates for

influence of factors, e.g. age, which could have

impact on the comparison of those rates

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SUMMARY

There are two methods: indirect and direct standardization

Indirect standardization applies age-specific rates from the standard population to the numbers of people in each age group in the index population

Direct standardization applies age-specific rates from the index population to the numbers of people in each age group of a standard population

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SUMMARY

Indirect standardization (calculation of SMR) does not require age-specific rates in the index population

Indirect standardization does not allow direct comparison between SMRs

Indirect standardization is more precise

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Health & Medicine

Standardization of rates