Measures of Occurrence Flashcards
What is meant by epidemiology?
The study of how often diseases occur in different groups of people and why
What is a key feature of epidemiology?
the measurement of disease outcomes in relation to a population at risk
What types of definitions are needed?
Clear definitions
e.g. “cancer” is not specific enough, need to consider sub-types
What must be defined?
the unit of measurement
e.g. a whole country, one hospital
What is meant by checking whether individuals are “eligible” for inclusion?
If someone moved from outside the study area with a pre-existing condition, they can be counted for prevalence but NOT incidence
How is disease occurrence usually expressed?
as a percentage
or a rate (e.g. number per 100,000 population)
What is the definition of an incidence of a disease?
the rate at which new cases occur in a population during a specified period
What is the calculation for incidence rate?
number of new cases in a period
/
number at risk in population in the period
What is essential to incidence?
It MUST include a time unit
e.g. 1 month, 5 years etc.
What is meant by prevalence?
the proportion of existing cases
this includes newly diagnosed cases and people who already have the condition
Why is prevalence useful for public health professionals?
It describes the burden of disease
What is meant by point prevalence?
The proportion of existing cases in a population at a single point of time
What is meant by period prevalence?
The number of new and existing cases over a set period e.g. one year
What is the only way that prevalence can be measured?
Cross-sectional surveys
How can prevalence be derived approximately?
by calculating:
incidence rate x average duration of disease
What is the difference between population and incident cases?
Population is the denominator population
Incident cases are new cases of the disease
What factor must be accounted for in the population?
In-migration
This is someone who has moved to the area but has not yet been diagnosed
What types of people will be in the prevalence pool?
The cases living in the population with the disease
This is incident cases, in-migration and the existing population
What needs to be excluded from the prevalence pool?
- out-migration
- deaths
- cures
Why is the denominator population constantly changing?
People are constantly moving in and out of the area
What is meant by mortality?
the incidence of death from a disease
the number of people dying from a disease in a given time period
What is the calculation for mortality rate?
number of people who die from the disease in a time period
/
number of people in the population in a time period
Where are details about individual deaths recorded?
on a death certificate
What is meant by ascertainment?
this means that you have counted all the individuals in your study
What can negatively affect ascertainment?
missing medical records
poor record keeping
changes in how diseases are classified might mean individuals might not be counted or the wrong ones might be included
What are examples of common of denominators?
- population of a country/region
- number of live births/stillbirths
- number of patients attending a hospital
- GP practice population
What would be an example of a denominator involving mortality rate in children?
mortality rate per 1000 live births
What is meant by crude rate?
a rate that refers to the population as a whole
What method is used to compare areas with different population structures?
standardisation
this is mainly by age and sex
What are the 2 methods of standardisation?
direct
indirect
What is involved in direct standardisation?
applying the rates of disease observed in the study group to a standard population
What is a standard population?
WHO standard populations contain data about people in different countries/world/etc.
What is involved in indirect standardisation?
applying the rates of disease in a ‘standard’ population to the study group
How is indirect standardisation usually presented?
it compares observed values to expected values as a ratio e.g. standardised mortality ratio (SMR)
What is the calculation for SMR?
observed number of deaths
/
expected number of deaths
