Week 2 - Measures Of Frequency And Rate Flashcards
Measures of disease frequency
Measures of disease frequency or measures of disease occurrence?
Assess the frequency and distribution of health conditions and health outcomes in populations
Often expressed s percentages since it reflects binary outcome (presence vs absence of health condition or occurrence vs no occurrence of health outcome)
Prevalence Equation
Point prevalence
The number of existing cases at a specific point in time divided by population at risk
‘Population at risk’
Individuals that are not immune to the disease, but normally whole population is used for ‘population at risk’
Period prevalence
Number of existing cases during a specific time period divided by population at risk.
More applicable for chronic diseases with either no cure or low mortality.
Prevalence
Point
Period
Incidence
Cumulative
Rate
Cumulative incidence
*The denominator is the same as for prevalence, but excludes existing cases occurring at the beginning of the time period.
Cumulative incidence
Also known as incidence proportion or incidence.
It is a cumulative measure as it quantifies all cases that appeared during a specific time period in relation to the population at risk.
Quantifies the risk of the outcome in a specific population at a specific time period
Application of prevalence and incidence
- Prevalence can display local, regional and/or global burdens of disease.
- Comparisons of disease frequencies between regions or subpopulation groups (e.g gender).
- Prevalence and incidence can be used to compare disease frequencies in different
population groups. - Regularly documented (ie seasonal or annual) incidence can inform about temporal or
time trends in disease patterns and/or burdens. - Diseases of emerging and/or projected public health concern.
All of the above applied to best inform clinical and public health practice.
Mortality rate
Mortality rate (also known as death rate) is the most important measure of
mortality and quantifies the number of deaths in a specific population during a
specified time period.
Different from incidence rate, but similar to cumulative incidence.
All-Cause Mortality Rate
- Also known as crude mortality (death) rate 2.Takes into account ALL deaths from ALL causes within a specified time point.
Cause-specific mortality rate
Cause-specific mortality rate
- Also known as cause-specific death rate
- Takes into account all deaths from a specific cause within a specified time point.
Age-specific mortality rate
- Also known as age-specific death rate
- Takes into account all deaths in a specific age-group within a specified time point
Case-fatality rate
Relationship between incidence, prevalence and mortality
How is prevalence determined?
Relationship between incidence, prevalence and mortality?
Prevalence = directly proportional to incidence
Prevalence = indirectly proportional to mortality
Cumulative Incidence
Incidence Rate
Incidence rate
- Also called rate
- It is based on the observation/reality that the population at risk keeps changing
over follow-up (i.e. it is not constant) as the people who develop the outcome or
who die from other causes, are not any more at risk.
Person - time
Time at risk for all individuals is calculated and added up
Why is incidence rate better than cumulative rate?
The main advantage of incidence rate, as compared to cumulative incidence, is
that it does not assume that all study participants are followed up for the whole
duration of a study.
Provides a more accurate denominator, hence a more accurate quantification of risk
What is the difference between incidence rate and cumulative rate?
Incidence rate can be interpreted in exactly the same way as cumulative
incidence; the only difference is the reporting of the per unit population.
Example:
* Cumulative incidence: 10 cases per 1,000 people in a given year
* Incidence rate: 10 cases per 1,000 person-years
Summary
