Measures of Frequency Flashcards
Explain ‘Prevalence’ and how it is calculated
Number of cases at one time point / Total population at risk at that time point
- Proportion shown as percentage
- USE: Cross-sectional studies
- Measures the number of existing cases in a population at one point in time
- Shows probability of being a case
- Shows burden of disease at the population level
Define Incidence
Measures the number of new cases in a population over a defined time period. Indicates risk of outcome occurring.
Explain Incidence Rate and how it is calculated
Number of new cases in a specified time period/Total person time at risk during that time period
- Expressed as [X cases] per [population] per [time period]
- Cohort studies and population level data
- Measures the frequency of outcome, accounting for variation in person-time at risk - used for open and dynamic populations
To estimate time at risk
Period mid-point x total period under consideration
Midpoint = # at start + # at end, divided by 2
Explain Risk and how it is calculated
Number of new cases in a specified time period/Number of individuals at risk at the START of that time period
- Shown as percentage
- Cohort study, individual level
- Measures the probability of an individual developing an outcome over a specified time period
Explain Odds and how it is calculated
Number of new cases in a specified time period/Number still at risk at END of that time period
- Normally expressed as a ratio eg. 1:3 (can be found via inverse of odds 1 / 0.33)
- Not usually used in epidemiology
- Measures the probability of becoming a case to not becoming a case
Can convert to risk: Odds / 1 + Odds
Explain Secondary Attack Rates
Number of new cases among contacts in a specified time period / Total number of contacts at risk during that time period
- Measures the probability that a contact of a primary case will develop the outcome during the specified time period
- Related to risk
- Proportion not a rate!
