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L7a - Measuring the Occurrence of Disease
Understand what we measure…
L7a - Measuring the Occurrence of Disease
- Understand what we measure disease frequency for and why;
- Describe the concepts of prevalence and incidence, risk and rates;
- Recognise how these measures are related;
- Calculate and interpret measures of disease frequency.
Epidemiology
The study of the distribution and determinants of health-related events/states in specified populations, and the application of this study to the control of health problems
- Identify who has the health related state (disease)
=> may also be social disease (domestic violence)
- Measure population disease frequency
- Predicting associations between an exposure and outcome
Person (characteristics), Place (social context), Time (chronological context) = Epidemiologist's mantra
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Types of Measures
Types of Populations
Closed (Fixed) Populations
- Defined by a specific event, thereby no new members
- E.g. People born in 1980 in Perth, Atomic Bomb survivors
- Loses members only through Death
- Population shrinks overtime
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Open (dynamic) populations
- New members introduced through various means
- Loses members through various means
E.g. Population
1. Count
• numerator only
• Eg, number of cases of food poisoning*
2. Ratio
- numerator and denominator
- numerator not necessarily contained in denominator
- Eg, number of stillbirths per 1000 livebirths*
3. Proportion
- special type of ratio
- numerator is contained in denominator
- Eg, number of infant deaths per total births*
4. Rate
- Ratio that involves time
- Eg, number of influenza cases per 1,000 population per year*
Prevelance
Prevalence = total number of cases in a given population at a specified point/period in time
- Represents the burden of disease in a population
- Represents a proportion not a rate (no indication of risk)
- P = (Total number of cases)/(Total population)
• Eg, 20% of persons have asthma at 30 June 2000
Point Prevalence
- Occurrence of disease at a specific point in time
- Eg, proportion of hospital patients with ‘pressure sores’ on 30 June 2012
Period Prevalence
- Occurrence of disease within a defined period of time
- Eg, proportion of UWA staff members who had a migraine headache during 2018.
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Incidence
Number of new cases of a disease that occur during a specified period of time in a population at risk for developing the disease
- Determines the RISK/RATE of developing a disease
1. Cumulative incidence
- Incidence proportion or ‘risk’
- Proportion of new cases of a disease occurring in a fixed population during a specified period of time
- CI = (Number of new cases) / (Number of people at risk) x (period of time that is being measured)
Estimates
- The risk of developing the disease during the specified period.
Assumes;
- The entire population have been followed for the entire specified time interval. (Doesn't account for attrition)
2. Incidence rate
- The numerator is the number of new cases in the population over the time period
- BUT the denominator is the sum of each individuals time at risk (ie, the time that each person stayed under observation and remained disease-free)
- Expressed as number of events per person-time (eg. person-day; person-month; person-year etc)
=> IR = (Number of people who get a disease in a specified period) / (total person - time at risk)-
Crtireria for determining constitutes a population at Risk (Incidence denominator)
- When a person is alive
- When they are able to contract the disease (age, gender etc)
- When they do not already have a disease
Similarities/Discrepancies Between CI and IR
- The more virulent a disease is the greater the discrimination between the two incidences are
- People acquire the disease early, thereby the time spent by those who never acquire the disease skews result of CI
- Generally the scores are similiar
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