View source for 16. Epidemiological indicators I: indicators of disease frequency and population impact of a disease

* Maths
** A fraction = number of cases / population
** A ratio = a number in relation to another one
** A proportion = the ratio of a part of the population to the total
** A rate = a proportion over time
* Indicators of disease frequency
** Number of cases
*** Simplest measure, but doesn’t take into account the size of the population
*** Useful for allocating health resources
** Incidence rate
*** New cases of a disease in the population over a period of time
*** Good for diseases with a short duration
*** Important when studying the etiology of the disorder
** Cumulative incidence rate
*** Proportion of new cases in the population over a period of time
*** Number of new cases over a period of time / population at risk over that period of time
*** For example: out of 5500 sex workers at risk for HIV in a 4-year period, 40 contracted HIV -> 40/5500 = 0,007 = 7%
*** Easy to interpret
** Incidence density
*** Number of new cases in the population / the number of time the population has spent at risk
*** The unit of the incidence density is “cases of disease/patient-year”
*** For example: 194 cases of breast cancer per 100 000 patient-years
*** Difficult to interpret
*** Gives a better estimate of true risk than the incidence
** Prevalence rate (often called just prevalence)
*** Measures all cases in the population, not just new cases
*** Period prevalence = number of all cases over a period of time / population at risk over that period of time
*** Point prevalence = number of all cases at a certain point in time / population at risk at that point in time
*** Lifetime prevalence = number of people who ever had the disease during their lifetime / population at risk at the beginning of the period
*** Good for permanent diseases, conditions, or risk factors
*** Gives an estimate of societal burden of the disorder
** Relationship between incidence and prevalence
*** A common disease with short duration, like the common cold, has a high incidence but low prevalence
*** A less common disease with a long duration, like diabetes type 2, has a low incidence but high prevalence
** Morbidity rate – when we measure cases of disease
** Mortality rate – when we measure death caused by disease
** Lethality (case fatality rate)
*** = number of people who died because of disease / number of people who had the disease
* Indicators of population impact of a disease
** These are all measures of disease burden
** Attributable risk (AR)
*** Shows the proportion of cases in exposed individuals which can be attributed to the exposure
*** The difference between the risk of disease in the population exposed to the risk factor, and the risk of disease in the non-exposed population
*** AR = incidence of exposed population – incidence of non-exposed population
*** Used in cohort studies
** Population attributable risk (PAR)
*** Shows how much of the incidence of a disease would decrease if the risk factor was eliminated
*** The difference between the risk of disease in the population as a whole, and the risk of disease in the population not exposed to the risk factor
*** PAR = incidence of whole population – incidence of non-exposed population
** Years of potential life lost (YPLL)
*** How many years a young person would live if they didn’t die prematurely
** Quality-adjusted life year (QALY)
*** Used to evaluate the value of public health interventions
** Disability-adjusted life year (DALY)
*** Includes both YPLL and QALY into one measurement
*** DALY = years lived with disability + years of life lost
** Numbers needed to treat

[[Category:Public health 2 - General epidemiology and demography]]