Week 1 Flashcards
What is epi? Measures of disease frequency: -Prevalence -Incidence Calculating incidence: -Risks -Rates Standardisation: -Direct -Indirect
What is epidemiology?
The study of the distribution and determinants of disease in human populations
Define prevalence:
Existing cases of an outcome of interest (e.g. disease), in a defined population at one point in time.
How is prevalence expressed?
As a proportion or percentage
Define point prevalence:
Prevalence literally at one point in time
Define period prevalence:
Prevalene measured over a (short) period of time (e.g. over the last year)
Define incidence:
Number of new cases of an outcome of interest (e.g. disease) arising from a defined population, during a time interval
How is incidence expressed?
As a rate (denominator has a time component)
Define risk:
Probability of disease occurring in a disease-free population during a specified time period
How is risk calculated?
Risk= n/P n= new cases in a defined period P= population at risk
What is a limitation of using risk as a measure?
Assumes that all of the population were followed up/at risk for the full time period, and ignores that follow-up periods for different people are often different.
Define rate:
Probability of disease occuring in a disease-free population during the sum of individual time periods.
How is rate calculated?
Rate= n/ total person-time of follow up
What is a benefit of calculating rate instead of risk?
Denominator makes explicit the time that the ‘population at risk’ spent being ‘at risk’.
What does person-time involve counting?
The exact time periods that participants were actually followed up for, added up together.
Explain cumulative incidence:
- Measured by risk
- Assumes that population-at-risk has been followed up for the entire specified period of time
- Used in closed cohort studies
Explain incidence density:
- Measured by rate
- Denominator expressed as person-time
- Used in open cohort study designs
What does crude risk/rate apply to?
The whole population (e.g. rate of heart disease in Australia)
What is category specific risk/rate?
Applies to a specific sub-population (e.g. rate of heart disease in Australian women aged 35-45 years)
What is one limitation of calculating crude rate?
Does not take into account differences in sub-population, e.g. age, which is a significant confounder for illness and death.
Define standardisation:
A process which adjusts for confounding factors (usually age) when comparing rates.
What does standardisation allow for?
The comparison of two populations, taking into account their different age distributions
What are the two types of standardisation?
Direct and indirect
What are the steps in direct standardisation?
- Stratify population by age
- Calculate age-specific rates to a reference population by age
- Apply age-specific rates to a reference population stratified by age
- Recalculate age-standardised rate
Explain the benefits and limitations of direct standardisation:
- Provides summary rates (all ages) that remove the unwanted effects of differences in the distribution of confounders in the population
- However the adjusted rates are not real
- Allows a fairer comparison after ironing out some of the other differences that may be exaggerating or masking differences in the population
- May involve more than two groups or populations
When is in-direct standardisation commonly used?
When age-specific rates are unavailable
What are the steps in indirect standardisation?
- Stratify study population by age
- Calculate expected number of deaths using standard rates applied to your population
- Calculate SMR (observed/expected)
- Recalculate age-standardised rate (crude rate of ref population by SMR)
Define and explain CMR:
Crude mortality rate:
CMR= (# deaths in population per time period/total population in time period) x 100,000
CMR depends on mortality rate in each age group, as well as proportion of people in age group in the population
Define and explain SMR:
Standardised mortality ratio:
SMR= (observed number of deaths/expected number of deaths) x 100%
Ratio tells us how the death rate in the study compares to the standard population
Allows for comparison of ‘health’ between populations
Define morbidity rate:
The incidence rate of non-fatal cases in the total population at risk during a specified time period
Define mortality rate:
The incidence rate of fatal cases in the total population at risk in a specified time period.
Mortality rate= number of deaths/total population
Define case fatality rate:
A measure of the severity of the disease.
Calculated by dividing number of deaths by all those who have the disease.
Define crude rate:
Rates that apply to an entire population without reference to any specific characteristic about the population.
Explain direct standardisation:
- Used to remove the biasing effect of age due to differing age structure of different populations
- Involves calculating the overall incidence or mortality you would have expected to find in a standard population if it had the same age-specific rates as your population
- Requires age-specific rates
- More commonly used than indirect
Explain indirect standardisation:
- Used if age specific rates are not available in the population requiring adjustment
- Uses rates derived from the standard population and applies them to the known age groups in the population to be standardised
