Het document bevat alle colleges en clips van het vak Study Design and Interpretation in Epidemiology and Public Health. Dit is een mastervak van de master Nutrition and Health aan Wageningen Universiteit.
HNH-31006 Study Design and Interpretation in Epidemiology and Public Health
Refresh knowledge
Cohort study
- Group of people followed over time until disease develops
- Purpose:
o To evaluate the occurrence (incidence) of disease in a carefully defined group of people
(monitoring)
o To investigate the causes of disease and to establish links between risk factors and health
outcomes (etiology)
- Measurements of exposure and outcome
o At baseline
▪ Exposure status
▪ Covariates: variables that are possibly related
with the exposure/outcome
o During follow-up
▪ Disease outcome
▪ Update information on exposure and covariates
▪ You count the number of incidence cases
Occurrence
- Count as incidence proportion or incidence rate
What is the exposure-disease association?
- Compare the occurrence of disease in people with and without exposure (etiology)
- We can compare incidence proportion or incidence rate in the exposed versus the unexposed
Example
- Calculate the Relative Risk (RR) of alcohol consumption on
cardiovascular disease
- RR = (10/(10+100))/(15/15+85)) = 0.61
Historical cohort study
- In a prospective cohort study the researcher starts with collecting the baseline data at the beginning
- In a retrospective cohort the researcher starts at the end
o Identify a population and observe events as they occur, and goes back to archives to find back
the previous exposure (determine exposure status from historical records)
o → requires good records of past exposure, such as birth weight
o → avoid long follow-up period, it gets more difficult to have the data present
,Strengths and limitations of a Cohort study
Strengths:
- Time sequence can be determined → causality
- Multiple outcomes /risk factors → sub analysis
- Study rare exposures
Limitations:
- Costs
- Not good for rare diseases
- Not good for diseases that take a long time to develop
- Ensuring people who started the study stay till the end
Case-control study design
Case-control study is very suitable:
- for rare conditions
- When disease slowly develop/have a long latency period
→ focus is usually on a single health outcome
→ much effort can be put on measuring the exposure of interest and potential confounders and on more
detailed measurements (e.g. interview, biomarkers (urine))
- Other advantages (though not always…)
o Less burden to participants
o Fast and cheap
The proper case group?
- Incident cases
o Newly diagnosed with the diseases
- Prevalent cases
o Both old and new occurrences
o Be aware of ‘reverse causation’ (similarly to cross-sectional studies)
- Be aware of the ‘supervisor effect’ (→ selection bias)
o Example: in a study of myocardial infarction, severe cases with highest exposure may die
before they reach the hospital and not enter the study
The proper control group?
- The control group should be representative for the (source) population of the cases
- Controls can be selected from general population, hospital, relatives, colleagues, friends
- Population controls
o Persons from the population where the cases came from who would also be identified as a
case when developing the disease
o Population registries
o Random digit dialling
o GP registrations etc
o Postal codes (neighbourhood controls)
Selection of controls
- Hospital controls = Patients admitted to the same hospital as the cases, but for other reasons (same
environment)
→ drawback: their risk factor distributions may not reflect that of the source population from which the cases
emerged
→ advantage: efficiency
Cross-sectional studies
- Measure all kind of exposures and all kind of diseases
- You can divide the population in people who are exposed to a certain factor and those who have the
disease where you are interested in
- Measures exposure and disease at the same moment in time
, Experimental study/trial
- The investigator assigns the exposure (randomly)
Epidemiological effect measures
Relationship between incidence, prevalence and disease duration
- Incidence = new cases
- Prevalence varies directly with both incidence and duration
o If incidence is low, and duration is long (chronic), prevalence will be large in relation to
incidence
o If the duration is short (due to recovery, migration or death),
prevalence will be small in relation to incidence
- Duration
Use of incidence and prevalence
1. Incidence is generally used for acutely acquired disease, prevalence is used for more permanent
states, conditions or attributes of ill-health
2. Incidence is more important when thinking of etiology of the disorder, prevalence when thinking of
societal burden of the disorder including the costs and resources consumed as a result of the disorder
Special types of incidence and prevalence measures
- Ways to express incidences: morbidity rate, mortality
rate, case-fatality rate, attack rate
- Ways to express prevalences: disease rate at autopsy,
birth defect rate
Incidence
- Counting new cases (e.g. 1 million)
- Measures how fast people are ‘catching’ the disease over a period in time
o E.g. in one month/year
- Expressed as:
o Proportion of people who develop the disease (e.g. 0.05 or 5%)
o Rate at which new cases of a disease have occurred (e.g. 5000 new cases per 100000 persons
per year)
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