What are the four components of measuring disease frequency?
- population (which group of people)
- case of disease (numerator)
- size of population (denominator)
- Time (be explicit)
what are the two types of populations? give examples.
- Fixed - a population defined based on some event; membership is PERMANENT
(people born in 2003 ) - Dynamic - a population whose membership is defined in a state or condition: membership is transient
(a resident of Houston - can move out etc)
Why should we distinguish between the two types of populations?
Informs which measures of disease frequency and which study designs are suitable
What are the 2 types of special populations?
- Catchment population: the population serviced by a medical facility
- Steady-state - describes a situation in which the number of people entering the population is equal to the number leaving
How can you measure generic disease frequency?
- Ratio: division of one number by another, the numbers do not need to be related
- Proportions: division of two related numbers, numbers must be related; the numerator is SUBSET OF the denominator, often expressed as a percentage
- Rate: division of one number by another in which TIME is an intrinsic part of the denominator, the most frequently misunderstood parameter
what are the three types of measures of disease frequency?
- prevalence
- cumulative incidence
- incidence rate
what is prevalence?
- measures the presence of EXISTING CASES of disease in a population during a specified period
- involves being in a state
- total population
why do we use prevalence?
- asses the burden of disease
(Administration and planning/Allocate resources) - Used for research when incidence is difficult to measure
=Conditions with uncertain onset (multiple sclerosis, depression)
what is point prevalence? example?
- the proportion of the population that has the disease at a single point in time
- On a date such as July 1st
what is incidence? two types?
- Measures occurrence of NEW CASES of disease in a population during a specified period
CI and IR
why do we use incidence?
Etologic research: what is causing this disease?
Evaluating prevention: Do they work to prevent new diseases?
Evaluating of treatments: do they work in improving survival and quality of life?
what is cumulative incidence? when should it be used?
- measures new cases in population at risk (need to mention time in explanation)
- Fixed populations with short follow-up or no loss to follow up
what critical assumption does CI make?
- All people in the population have been followed for the entire specified time period - No loss to follow-up
what are the two types of CI? formula?
- attack rate: proportion of individuals exposed to an infectious agent who becomes infected over a certain period (New case/ population at risk at the beginning of the period)
- Case fatality “rate” is the proportion of individuals with a disease who die of the disease (Number deaths due to disease X/ number with disease X )
what are the limitations and strengths of CI?
Limitations
- Assumes that the people being observed have been followed for the entire period of observation
- CI is not a perfect measure in a dynamic population or a fixed population that loses members over time
- Does not consider the time of occurrence - does take into account WHEN the disease occurs
Strength
- easy to calculate
- used to estimate individual risk (like 1 in 8 women)
what is the incidence rate? different names? when should it be used?
- occurrence of new cases of disease that arise during person-time of observation
- true rate, incidence density
- in Dynamic or fixed populations
what is personal time?
- Amount of time each person at risk is under observation
- Accrued only among people being followed up
- does not include people who leave, die, develop disease
what are the limitations and strengths of IR?
Limitations
Not easily calculated
Can be difficult to figure out person-time
Interpretation is not intuitive
Strength
Does not assume complete follow-up
Can take into account when a disease occurs
What are the three characteristics shared by P/IR/CI?
Number of cases (numerator)
Size of population (denominator)
Measure of time
What is the relationship between prevalence and incidence? formula? assumes?
P≈ IR * D
D = average duration of disease; this is the time from diagnosis to recovery or death
Assumes:
Population is in a steady state where inflow equals outflow
Prevalence is low (<10%)
D ≈ P/IR
This formula is used to estimate the average duration of disease
crude mortality rate
- Total number of deaths from all causes per 100,000 population per year.
cause-specific mortality rate
Number of deaths from a specific cause per 100,000 population per year.
age-specific mortality rate
otal number of deaths from all causes among individuals in a specific age category per 100,000 population per year in the age category.
morbidity rate
Number of existing or new cases of a particular disease or condition per 100 population.
