Population Surveys Methodologic problems: standardization Maura Pugliatti, MD, PhD Associate Professor of Neurology Dept. of Clinical and Experimental Medicine, Unit of Clinical Neurology University of Sassari, Italy 1 st International Course of Neuroepidemiology Chisinau, Moldova, Sept. 2012

Personal characteristics AgeOccupation Year of BirthEducation SexIncome Marital statusEthnic 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.

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 M r by P s. 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" SDR 1 = [Sum age groups (M r x P s )]/P t x 1000 M r is the age-specific mortality rate for the region. P s is the number of people in the age group in the standard population. P t is the total standard population.

Indirect Standardization: To compute the indirect standardized mortality rate: 1. For each age group, you need to multiply M s by P r. 2. Add them up. 3. Divide the sum into the number of deaths in the region. 4. Multiply by the crude death rate. SDR 2 = D r /[Sum age groups (M s P r )] x CDRs M s is the age-specific mortality rate for the standard population. P r is the number of people in the age group in the region’s population. D r is the number of deaths in the region. CDR s is the crude death rate for the standard population.

DIRECT METHOD OF STANDARDIZATION Examples

SDR 1 = [Sum age groups (M r x P s )]/P t x 1000 Example: M r is the age-specific mortality rate for the region. P s is the number of people in the age group in the standard population. P t is the total standard population

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

Prevalence of ALS in northern Sardinia (prov. of Sassari and Nuoro) in 2004 Standard population 2004 census pop. (denominator) ITALY Standardized ALS prevalence in northern Sardinia 8,9 5,1 6,9 13,1 8,2 10,610, 4 6,2 8,3 Crude prevalence Pugliatti M, Parish LD, et al. J Neurol, in press

Gallitto et al. Epilepsia, Prevalence and Characteristics of Epilepsy in the Aeolian Islands

1999 census pop. (denominator) Northern Sardinian population Age-specific prevalence of epilepsy in Aeolian Islands, 1999 Gallitto et al, Epilepsia 2005 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.

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DIRECT METHOD OF STANDARDIZATION Other Examples

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

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

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

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