population types
Closed
Fixed - Membership permanent, ends only by death
defined by a specific event
Open
Dynamic) - Membership changes over time (migration, births, deaths)
no single defined event
how do we measure disease in a population (the basis)
numerator: case count
denominator: population size
time: follow up
ratio
Relationship between two numbers (one number divided by the other)
numerator and denominator can be different groups
proportion
type of ratio where the numerator is part of the denominator
rate
a proportion with a time component
-num: number events in a period
-denom: total person time at risk
describe what person time is
the sum of the time periods that each individual in a study is observed and at risk for developing the outcome.
a person is at risk until they either develop the disease or the study period ends
prevalence
Proportion of the total population with the condition at a specific
time or over a period
point prevalence vs period prevalence
Point prevalence: snapshot of the situation at a single point in time
Period prevalence: the proportion of the population that has the disease at any time during a specified time period
incidence
new cases/onset in a time period
measures how quickly people are getting the disease
a rate
Diagnosis of a disease is based off of;
Symptoms: felt by the person
Signs: seen by a clinician
Tests: additional testing may be necessary
○ This can be helpful for some things, but not as objective for other diseases like depression
prevalence is the main outcome in what kind of study
cross-sectional
prevalence is not good for
determining causality
factors that increase prevalence
Longer disease duration
* Prolongation of life without cure
* Increased in-migration of cases
* In-migration of susceptible people
* Out-migration of healthy people
* Improved diagnosis/reporting
factors that decrease prevalence
- Shorter disease duration
- High case fatality
- Decreased incidence
- In-migration of healthy people
- Out-migration of cases
- Improved cure rates
what kinds of studies does incidence need
cohort, to be able to follow up over time
incidence is good for
causal/etiological inference
if P<10% the disease is
rare
rate ratio
tells us how many times higher the rate of disease is in one group compared to another
§ Strength of association
rate difference
tells us how much extra disease occurred in one group compared with another group
§ Gives some sense of potential value of a preventative intervention
Attributable Risk (AR)
How much of the disease among the exposed is due to the
exposure?
Odds ratio
Odds: Calculated as the likelihood of the event happening to it not happening
Excess risk:
A term used when exposure is harmful. If RD is negative, “excess risk”
does not apply
risk is a
proportion
a risk ratio measures
strength of association between exposure and outcomes
