Lecture 4: Measures of occurrence Flashcards
(32 cards)
Epidemiology is the study of how often diseases occur in different groups of people and why. A key feature of epidemiology is the measurement of disease outcomes in relation to a population at risk.
- Clear definitions are needed (‘cancer’ is not precise - consider subtypes - Define your unit of measurement (e.g. whole country, region) - Check that individuals are ‘eligible’ for inclusion
What are the different ways that a disease/death can be described?
- Simple percentages 2. Rates
What is the incidence of a disease?
The incidence of a disease is the rate at which new cases occur in a population during a specified period
E.g. If population is 1000 but 200 have already had disease, the population at risk is only 800
How is the incidence rate calculated?
Incidence rate = number of new cases in period / number at risk in population MUST include time unit
What is ‘prevalence’ defined as?
Defined as the proportion of existing cases (old and new)
Be careful - it is not the risk (or rate) of disease
When is prevalence useful?
Useful for public health professionals and commissioners - describes the burden of disease.
What is ‘point prevalence’?
The proportion of existing cases in a population at a single point of time
How can point prevalence be approximately calculated?
By calculating incidence rate x average duration of disease
Point prevalence is the only measure obtainable from cross-sectional studies. What is a cross-sectional study?
A is a type of observational study that analyzes data from a population, or a representative subset, at a specific point in time
Incidence and prevalence explained - think of it like a funnel.
How should in-migration, out-migration, death and cures be dealt with?
Population - this is your denominator population
Incident cases - these are new cases
Out-migration, deaths and cures must be exclused (population/cases will decrease)
In-migration must be included (population/cases will increase)
What is in-migration?
The process of people moving into a new area in their country to live there permanently
I.e. if people with the disease move there, you have to count them in the population
What is out-migration?
The process of people moving out of an area in their country to move to another area in their country permanently.
Prevalence vs incidence graph
Example rates:
- 18.3/100,000 for non-South Asians
- 13.1/100,000 for South Asians
BUT these are rate for a 16 year period and rates can change over time
Age-specific rates example
Different conditions may be more prevalent in different age groups
Incidence rate example
What is the definition of mortality?
Mortality is the incidence of death from a disease
How is mortality rate calculated?
Mortality rate = number of people who die from the disease in period / number of people in the population in period
E.g. 200 / 20000 (200 deaths in population of 20000) in same period
What does ‘ascertainment’ mean?
Means that you have counted all the individuals of interest in your study
Missing medical records, poor record keeping or changes in how diseases are classified might mean don’t count individuals, or include the wrong individuals
When might ‘ascertainment bias’ arise?
Arises when data for a study or an analysis are collected (or surveyed, screened, or recorded) such that some members of the target population are less likely to be included in the final results than others.
Poor ascertainment example:
If you wanted to use death certificates to find out the cause of death in children and young people diagnosed with Type 1 diabetes
- Out of 229 death certificates examined only 54% of all death certificates mentioned diabetes and 21% specifically mentioned Type 1 diabetes
- If you JUST used ONS death certificate data you would MISS nearly half of all deaths in this group – that is poor ascertainment
Common denominator examples:
- Population of a country/region – census data
- Number of live births/live births and stillbirths/live births, still births and TOPs – ONS
- Number of patients attending a hospital – Hospital episode stats, individual hospital clinical information systems
- A GP practice population, data from their primary care database provider
When looking at a disease you might assess incidence over a long period of time (e.g. 10 years). How does this affect the persons chances of getting the disease?
This means that your population has had a longer time at risk of getting the disease.
In a population of 1,000,000 people over 10 years, their person years at risk is 10,000,000, BUT populations change
What is a crude rate?
The total number of events, or count, divided by the mid-year total population of the selected geography and multiplied by a constant, which is a multiple of 10. Typical constants used for public health rates include 100, 1,000, 10,000, or 100,000.
Crude rate relates to the population as a whole
