Population Surveys Methodologic problems:

standardization

Maura Pugliatti, MD, PhD

Associate Professor of NeurologyDept. of Clinical and Experimental Medicine, Unit of Clinical Neurology

University of Sassari, Italy

1st International Course of NeuroepidemiologyChisinau, Moldova, 24-28 Sept. 2012

Personal characteristics

Age Occupation

Year of Birth Education

Sex Income

Marital status Ethnic group/Religion

Time

Short term variations

Long term variations (secular changes)

Place

International variations

Regional variations

Local variations

Urban/Rural variations

Disease determinants: factors of importance for disease occurrence

Gallitto et al. Epilepsia, 2005.

Standardization

Standardization allows for comparisons when the population structures differ and is key in assessing the potential influence of environmental or cultural factors on disease rates in a region.

A standardized estimate is a crude estimate that has been adjusted for differences in age composition between the study population and a standard (referral) population.

Standardized estimate based on the same standard population are comparable and the ratio between two standardized estimates is called Standardized Rate Ratio (SRR)

Standardization for gender and age

There are two ways of computing standardized estimates: -Direct Standardization: it calculates a weighted average of the region’s age-specific estimates where the weights represent the age-specific sizes of the standard population.

-Indirect Standardization: it uses age-specific estimates (eg., mortality) from the standard population to derive expected estimates (eg., deaths) in the region’s population.

Choosing which formula to use will depend on what data you have access to.

AgePopulation

USMortality New

YorkMortality Arizona

0-4 18456 0,00272 0,002695-24 72053 0,00066 0,0007625-34 43675 0,00192 0,0014635-44 35264 0,00321 0,0022445-54 24163 0,0054 0,0048455-64 21830 0,01212 0,0110265-74 17897 0,02793 0,0240875+ 12470 0,08753 0,07359

Table 2, U,S, Age-Specific Mortality and State Populations in Thousands Age US Death New York Arizona

Rate Population Population0-4 0,00251 1275 2995-24 0,00065 5023 102825-34 0,00135 3018 61435-44 0,0022 2572 47945-54 0,00486 1909 31555-64 0,01236 1784 30765-74 0,0273 1347 27775+ 0,08513 981 170

Table 1, US Population in thousands and State Age-Specific Mortality Rates

Direct Standardization(eg., incidence, prevalence, mortality)

Indirect Standardization(eg., mortality)

Type of data we can have access to:

Direct Standardization:

To compute the direct standardized mortality rate (SMR):

1. For each age group, you need to multiply Mr by Ps.

2. Add them up.

3. Divide the sum by the total standard population.

4. Multiply by 1000, or give the rate in terms of "per thousand population"

SDR1 = [Sum age groups (Mr x Ps)]/Pt x 1000

Mr is the age-specific mortality rate for the region.Ps is the number of people in the age group in the standard population.Pt is the total standard population.

Indirect Standardization:

To compute the indirect standardized mortality rate:

1. For each age group, you need to multiply Ms by Pr.

2. Add them up.

3. Divide the sum into the number of deaths in the region.

4. Multiply by the crude death rate.

SDR2 = Dr/[Sum age groups (Ms Pr)] x CDRs

Ms is the age-specific mortality rate for the standard population.Pr is the number of people in the age group in the region’s population.Dr is the number of deaths in the region.CDRs is the crude death rate for the standard population.

DIRECT METHOD OF STANDARDIZATION

Examples

SDR1 for New York = ???

SDR1 for Arizona = ???

SDR1 = [Sum age groups (Mr x Ps)]/Pt x 1000

Example:

Mr is the age-specific mortality rate for the region.Ps is the number of people in the age group in the standard population.Pt is the total standard population

AgePopulation

US

Mortality New

York

Mortality

ArizonaNY AZ

0-4 18456 0,00272 0,00269 50,20 49,655-24 72053 0,00066 0,00076 47,55 54,7625-34 43675 0,00192 0,00146 83,86 63,7735-44 35264 0,00321 0,00224 113,20 78,9945-54 24163 0,0054 0,00484 130,48 116,9555-64 21830 0,01212 0,01102 264,58 240,5765-74 17897 0,02793 0,02408 499,86 430,96

75+ 12470 0,08753 0,07359 1091,50 917,67

Tot 245808 2281,23 1953,31

Table 1, US Population in thousands and State Age-Specific Mortality Rates

SDR1 for New York = 9,3

SDR1 for Arizona = 7,9

M F T M F T M F T<35 97725 93407 191132 56678 53519 110197 154403 146926 301329

35-44 37116 37579 74695 20607 19978 40585 57723 57557 11528045-54 31494 31413 62907 17787 17189 34976 49281 48602 9788355-64 26850 28367 55217 14750 15798 30548 41600 44165 8576565-74 19544 23266 42810 11653 14247 25900 31197 37513 6871075-84 10467 15599 26066 6700 9962 16662 17167 25561 4272885+ 2788 5069 7857 1846 3279 5125 4634 8348 12982Tot 225984 234700 460684 130021 133972 263993 356005 368672 724677

SS 2004 NU 2004 TOT 2004

M F T M F T M F T1 1 2 0 0 0 1 1 20 1 1 2 0 2 2 1 34 2 6 5 0 5 9 2 118 5 13 5 1 6 13 6 194 3 7 4 7 11 8 10 183 0 3 1 3 4 4 3 70 0 0 0 0 0 0 0 020 12 32 17 11 28 37 23 60

SS 2004 NU 2004 TOT 2004

M F T M F T M F T1,0 1,1 1,0 0,0 0,0 0,0 0,6 0,7 0,70,0 2,7 1,3 9,7 0,0 4,9 3,5 1,7 2,612,7 6,4 9,5 28,1 0,0 14,3 18,3 4,1 11,229,8 17,6 23,5 33,9 6,3 19,6 31,3 13,6 22,220,5 12,9 16,4 34,3 49,1 42,5 25,6 26,7 26,228,7 0,0 11,5 14,9 30,1 24,0 23,3 11,7 16,40,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,08,9 5,1 6,9 13,1 8,2 10,6 10,4 6,2 8,3

SS 2004 NU 2004 TOT 2004

Prevalence of ALS in northern Sardinia (prov. of Sassari and Nuoro) in 2004

2004 ALS prevalence

2004 number of ALS cases

2004 census pop. (denominator)Sardinian provinces of Sassari and Nuoro

Pugliatti M, Parish LD, et al. J Neurol, in press

The island of Sardinia, Italy

M F T WeightM WeightF WeightT

<35 11714767 11247020 22961787 0,4173619 0,3771682 0,39665735-44 4612114 4579893 9192007 0,1643157 0,1535865 0,15878945-54 3761710 3851041 7612751 0,1340184 0,1291445 0,13150855-64 3383481 3609738 6993219 0,1205432 0,1210524 0,12080665-74 2737383 3282765 6020148 0,0975247 0,1100874 0,10399675-84 1526720 2433082 3959802 0,0543924 0,0815933 0,06840485+ 332433 816098 1148531 0,0118436 0,0273678 0,01984Tot 28068608 29819637 57888245 1 1 1

M F T M F T M F T0,4 0,4 0,4 0,0 0,0 0,0 0,3 0,3 0,30,0 0,4 0,2 1,6 0,0 0,8 0,6 0,3 0,41,7 0,8 1,3 3,8 0,0 1,9 2,4 0,5 1,53,6 2,1 2,8 4,1 0,8 2,4 3,8 1,6 2,72,0 1,4 1,7 3,3 5,4 4,4 2,7 2,9 2,71,6 0,0 0,8 0,8 2,5 1,6 1,6 1,0 1,10,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,09,3 5,2 7,2 13,6 8,6 11,1 11,2 6,6 8,7

TOT 2004SS 2004 NU 2004

Prevalence of ALS in northern Sardinia (prov. of Sassari and Nuoro) in 2004

Standard population2004 census pop. (denominator)ITALY

Standardized ALS prevalence in northern Sardinia

8,9 5,1 6,9 13,1 8,2 10,6 10, 4 6,2 8,3 Crude prevalence

Pugliatti M, Parish LD, et al. J Neurol, in press

Gallitto et al. Epilepsia, 2005.

Prevalence and Characteristics of Epilepsy in the Aeolian Islands

Prevalence and Characteristics of Epilepsy in the Aeolian Islands

M F T WeightM WeightF WeightT

5-14 42357 39261 81618 0,188267 0,170759 0,17941815-24 40421 38737 79158 0,179662 0,16848 0,1740125-34 36087 36298 72385 0,160398 0,157872 0,15912135-44 31843 31191 63034 0,141535 0,13566 0,13856545-54 26647 27514 54161 0,11844 0,119668 0,1190655-64 22230 23907 46137 0,098807 0,10398 0,10142165-74 14707 17861 32568 0,065369 0,077684 0,07159375+ 10692 15151 25843 0,047523 0,065897 0,05681Tot 224984 229920 454904 1 1 1

1999 census pop. (denominator)Northern Sardinian population

M F T5–14 5,01 5,09 5,0515–24 0 2,56 1,2525–34 2,9 3,03 2,9635–44 4,27 4,58 4,4245–54 2,99 4,63 3,7555–64 1,41 0 0,7365–74 6,76 4,27 5,41>74 0 1,99 1,24Total 2,95 3,3 3,13

Age-specific prevalence of epilepsy in Aeolian Islands, 1999Gallitto et al, Epilepsia 2005

M F T5–14 0,94 0,87 0,9115–24 0,00 0,43 0,2225–34 0,47 0,48 0,4735–44 0,60 0,62 0,6145–54 0,35 0,55 0,4555–64 0,14 0,00 0,0765–74 0,44 0,33 0,39>74 0,00 0,13 0,07Total 2,95 3,29 3,19

Standardized prevalence of epilepsy to the 1999 Sardinian pop.

Gallitto et al. Epilepsia, 2005.

per 1000

Prevalence and Characteristics of Epilepsy in the Aeolian Islands

Gallitto et al. Epilepsia, 2005.

M F TCrude prevalence 2,95 3,30 3,13Stand. Prev to 2001 Italian pop. 2,87 3,15 3,01Stand. Prev to 1999 Sardinian pop. 2,95 3,29 3,19

Gallitto et al. Epilepsia, 2005.

**

DIRECT METHOD OF STANDARDIZATION

Other Examples

Example:

Pop. Deaths Mortality Pop. Deaths Mortality

0-29 yrs 3145000 3523 1,1 741000 3904 5,330-59 yrs 3057000 10928 3,6 275000 1421 5,260+ yrs 1294000 59104 45,7 59000 2456 41,6Total 7496000 73555 9,8 1075000 7871 7,3

Sweden Panama

0-29 yrs 3145000*5,3/1000 16569,630-59 yrs 3057000*5,2/1000 15796,460+ yrs 1294000*41.6/1000 53865,5Total 86231,5

SMR: 86231,5*1000/7496000 11,5SRR 11.5/9.8 1,17

Direct standardization

The choice of a standard population affects the comparison.

Here the Sweden population is used as standard, the Panama rates applied to this population

Example:

Pop. Deaths Mortality Pop. Deaths Mortality

0-29 yrs 3145000 3523 1,1 741000 3904 5,330-59 yrs 3057000 10928 3,6 275000 1421 5,260+ yrs 1294000 59104 45,7 59000 2456 41,6Total 7496000 73555 9,8 1075000 7871 7,3

Sweden Panama

Here the Panama population is used as standard, the Swedish rates applied to this population

0-29 yrs 7411000*1.1/1000 830,130-59 yrs 275000*3.6/1000 983,160+ yrs 59000*45.7/1000 2694,9Total 4508,0

SMR: 4508*1000/1075000 4,2SRR 7.3/4.2 1,75

Direct standardization

0-9 10-19 20-39 40+ Tot

Study A N. Cases 62 280 131 126 599At risk population 10500 22500 25000 42000 100000Prevalence (per 1000) 5,9 12,4 5,2 3,0 6,0

Study B N. Cases 165 538 69 68 840At risk population 24500 42000 12500 21000 100000Prevalence (per 1000) 6,7 12,8 5,5 3,2 8,4

35000 64500 37500 63000 200000

Standardized prevalence

Study A 7,0 per 1000 =(5.9*35000)+(12.4*64500)+(5.2*37500)+(3.0*63000)/200000Study B 7,4 per 1000 =(6.7*35000)+(12.8*64500)+(5.5*37500)+(3.2*63000)/200000

Age groupsEpilepsy: crude data from two studies

Standard population (sum of A+B)

Direct Method of Standardization

Rosati G, Granieri E. Manuale di Neuroepidemiologia Clinica, 1990

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