PUBLIC HEALTH EXTRAS Flashcards
EPIDEMIOLOGY
Define epidemiology. What factors are considered when measuring epidemiology of a disease?
The study of the frequency, distribution + determinants of disease + health-related states in populations in order to prevent + control disease.
- Time, place, person (age, gender, class, ethnicity).
EPIDEMIOLOGY
Define incidence.
The number of new cases of a disease that develop in a population (e.g. per 100,000) in a given time frame (e.g. per year).
EPIDEMIOLOGY
Define prevalence.
The total # of people in a population found to have a disease at a point in time.
- Number of existing cases/population/points in time.
EPIDEMIOLOGY
What is meant by person-time?
The measure of time at risk i.e. time from entry to a study to:
- Disease onset.
- Loss to follow up.
- End of study.
It is the sum of each individual’s time at risk (i.e. length of time they were followed up in the study).
EPIDEMIOLOGY
How do you calculate incidence rate calculations?
of persons who have become cases in a given time period ÷ total person-time at risk during that period.
- Person-time is the denominator in them.
EPIDEMIOLOGY
What is the denominator in cumulative incidence calculations?
Number of disease-free at start of study.
EPIDEMIOLOGY
What is meant by:
i) an independent variable?
ii) a dependent variable?
i) A variable that can be altered in a study.
ii) A variable that is dependent on the independent variables or one that cannot be altered.
EPIDEMIOLOGY
What is meant by absolute risk?
Gives a feel for actual numbers involved i.e. it has units.
- E.g. deaths/1000 population.
EPIDEMIOLOGY
What is meant by relative risk?
Ratio of risk of disease in the exposed to the risk in the unexposed i.e. no units.
- It tells us about the strength of association between a risk factor + a disease.
EPIDEMIOLOGY
How do you calculate relative risk?
Incidence in exposed ÷ incidence in unexposed.
EPIDEMIOLOGY
What is meant by attributable risk?
The rate of disease in the exposed that may be attributed to the exposure.
- Attributable risk is a type of absolute risk (absolute excess risk).
- It’s about the size of effect in absolute terms i.e. gives a feel for the public health impact (if causality assumed).
EPIDEMIOLOGY
How do you calculate attributable risk?
Incidence in exposed – incidence in unexposed.
EPIDEMIOLOGY
What is meant by relative risk reduction?
The reduction in rate of the outcome in the intervention group relative to the control group.
EPIDEMIOLOGY
How do you calculate relative risk reduction?
(Incidence in unexposed – incidence in exposed) ÷ incidence in unexposed.
- (ARR ÷ incidence in unexposed).
EPIDEMIOLOGY
What is meant by absolute risk reduction?
The absolute difference in the rates of events between the 2 groups. Gives an indication of the baseline risk + the intervention effect.
EPIDEMIOLOGY
Give an example of absolute risk reduction?
Assuming exposed means they have had a particular intervention (such as giving statins to people with hypercholesterolaemia) + then a control group who do not have statins + seeing how many in each group have a heart attack to see if the intervention of statins is effective.
EPIDEMIOLOGY
How do you calculate absolute risk reduction?
Incidence in unexposed – incidence in exposed.
EPIDEMIOLOGY
What is meant by odds?
The odds of an event is the ratio of the probability of an occurrence compared to the probability of a non-occurrence.
EPIDEMIOLOGY
How do you calculate odds?
Probability ÷ (1 – probability).
EPIDEMIOLOGY
What is meant by odds ratio?
The ratio of odds for the exposed group to the odds for the non-exposed groups.
- Or can be interpreted as a relative risk when the event is rare.
EPIDEMIOLOGY
When would odds ratio be used?
For case control studies as it’s not possible to calculate the relative risk.
- For cross-sectional + cohort studies – both can be derived but odds ratio is used if it’s not clear which is the independent/dependent variable.
EPIDEMIOLOGY
How do you calculate odds ratio?
ODDS EXPOSED / ODDS UNEXPOSED
(P exposed ÷ [1 – P exposed]) ÷ (P unexposed ÷ [1 – P unexposed])
EPIDEMIOLOGY
What is meant by number needed to treat?
The number of patients that need to be treated in order to prevent one bad outcome.
EPIDEMIOLOGY
How do you calculate NNT?
1 ÷ ARR (risk in unexposed – risk in exposed)
