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Population Surveys Methodologic problems: standardization

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### Population Surveys Methodologic problems: standardization

Maura Pugliatti, MD, PhDAssociate Professor of NeurologyDept. of Clinical and Experimental Medicine, Unit of Clinical NeurologyUniversity of Sassari, Italy 1st International Course of Neuroepidemiology

Chisinau, 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

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.

Type of data we can have access to:

Direct Standardization

(eg., incidence, prevalence, mortality)

Indirect Standardization

(eg., mortality)

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.

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 = [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

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

The island of Sardinia, Italy

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

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

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

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

Prevalence and Characteristics of Epilepsy in the Aeolian Islands

Gallitto et al. Epilepsia, 2005.

Prevalence and Characteristics of Epilepsy in the Aeolian Islands

Gallitto et al. Epilepsia, 2005.

Prevalence and Characteristics of Epilepsy in the Aeolian Islands

per 1000

Gallitto et al. Epilepsia, 2005.

DIRECT METHOD OF STANDARDIZATION

Other Examples

The choice of a standard population affects the comparison.

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

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

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