- Why study mortality statistics
o vital statistics
- Source of mortality data
o death certificates
o Australian Institute of Health and Welfare
- Problems with death certificate
o problem assigning one cause of death
§ death coded according to underlying cause of death
§ excludes info on immediate cause of death & inter mediate conditions
o problem with accuracy and completeness of information provided.
§ medical opinions might differ.
§ some diseases under-reported à AIDS
§ some diseases over-reported à stroke
§ differences amongt medical practitioners in classification
§ international differences
§ use of International Classification of Disease codes
· revised periodically
- other sources of mortality data
o autopsy
o hospital records
o occupational records
o insurance / pension fund records
- mortality studies
o difference in mortality trends over time / between populations
§ may be artifactual à result of errors in numerator or denominator
§ may be real
o Artifactual
§ Errors in numerator
· changes in recognition of disease
o earlier detection of disease
· changes in coding rules
· changes in classification
§ Errors in denominator
· errors in counting population
o eg, under representation of young black males
· errors in classifying by demographic characteristics
o Real
§ changes in incidence of disease
· genetic
· environmental
o EG, DECRAESED MORTALITY FROM ECTOPIC PREGNANCY
· changes in age distribution of population
o child birth – occurs in women of child bearing age, disappears as women reach menopause.
- mortality statistics
o rates
§ two denominators
· mid year population
· person years
o total amount of time for which people were observed to be disease free during time of followup.
o crude and specific rates
§ crude mortality rate (CMR) / Crude Death Rate (CDR)
· all deaths during calendar year / mid year population
· advantages
o simple to understand & calculate
o widely used
o is a probability rate that a person belonging to population will die.
· disadvantages
o ignores age and sex distribution of population
o specific mortality rates (SMR) / Specific Death Rate (SDR)
§ subdivision of data into homogenous subgroups
§ advantages
· take into account age / sec composition of population
· widely used
· supplies essential components for construction of life table
§ disadvantages
· not useful for comparing rates in different regions
· in addition to age and sex distribution of population, social, occupational and topographical factors cause differential mortality
o to avoid such spurious effects, standardized death rates are calculated.
o cause specific mortality rate (CSMR)
§ common conditions à cancer, CHD
§ compare different populations
§ number of deaths specific cause / population
o maternal mortality ratio
§ measure of risk of dying from puerperal causes – assoc with pregnancy
§ up to 42 days after termination / completion of pregnancy
· puerperal deaths / live births
o Infant mortality rate
§ deaths of infants under one year of age / number of live born infants
- Age affects the rate
o differences in age composition
o crude rates à do not take into account age differences
o versus adjusted / specific rates
- standardized rates / adjusted rates
o adjusted to control for effects of age
o direct standardization
§ calculate age specific mortality rates for each age group in each population
§ select population whose age distribution is well defined to serve as standard or reference population
§ multiply number of people in each age group of reference population by age specific mortality rate in populations of interest.
§ sum total number of expected deaths
§ divide total number of expected deaths by total number of people in reference population
· to yield summary age adjusted mortality rate.
o Indirect Standardization
§ used when either age specific mortality rates not available
§ or they are statistically unstable
· when population to be standardized is small.
§ SMR = observed number of deaths / expected number of deaths
§ example – are death rates for male workers at certain company similar to death rate in population
· choose reference population
o make sure age specific mortality rates are known in reference population
· calculate observed number of deaths in populations of interest
· multiply age specific death rate from each age group in reference population by number of workers in corresponding age groups in company
· sum the total number of expected deaths
· divide total number of observed deaths in pop of interest by expected deaths
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