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
- Number of cases
- 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