- Rates- how fast the disease is occurring in a population
- Proportion – what fraction of population is affected
- prevalence
o numerator includes mix of people with different durations of disease
o hence, do not have a measure of risk
o measure of burdon of disease in community
- problems with incidence and prevalence measures
o problem with numerator
§ defining who has the disease
§ ascertaining which people should be included in numerator
· how do we find cases
o problem with denominator
§ selective undercounting of certain groups in pop – young men in ethnic minorities
§ different ways to classify by ethnic group
§ example
· for rate to make sense, people represented by denominator must have potential to enter group represented by numerator
o hysterectomy / uterine cancer rates
o problems with hospital data
§ admissions policies
§ records not design for research
§ if we wish to calculate rates, have a problem defining denominator
· hospitals do not have defined catchment area.
- Relationship between incidence and prevalence
o in a steady state situation, in which rates are not changing and in – migration equals out migration
§ prevalence = incidence * duration
o example
§ extramarital births as percentage of total births in NZ
· increased, but only because marital births decreased.
o proportion is not a rate
o birth can be viewed as an event
o malformations à prevalence at birth
o breast cancer incidence
§ distribution of cases by age
§ oldest age group has highest risk of breast cancern, but because this group is so small, only contribute small proportion of total number of breast cancern cases.
o spot maps
§ clustering on spot map does not demonstrate higher incidence in area of cluster, for population most often clusters in that area.
§ many apparent clusters due by chance
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