4. Descriptive epidemiology Flashcards
Target population =
People you want to apply your results to
(e.g. everyone living in australia)
Source population/sampling frame =
People from whom the population is selected
(e.g. everyone on autralian election roll)
Sample =
People approached tot ake part in the survey
(e.g. random sample of those listed)
Study population =
People who actually took part in the survey
What is the real strength of randomization?
- The real strength of randomisation is that, on average, it will also balance these other unknown or poorly measured factors across the groups.
Why is it important to have large groups in an RCT?
If the groups are small it is unlikely that all of the factors that could affect the outcome will be evenly distributed across the groups.
Of non-randomised designs, the most common are historical controls.What happens in such a design? What are these designs also called? Name an example.
> health outcomes after introduction of a new treatment/preventive measure are compared to the outcomes experienced by the same population before the change in practice
(also sometimes called a pre–post study)
e.g. patient survival rates might be compared before and after the introduction of a new surgical technique
What is a major problem with historical controls?
Assumes that the only (or most important) thing that has changed is the new legislation or the type of surgery, and that may not be the case.
Why must participants be free of the outcome of interest at the start of the follow-up of a prospective/longitudinal cohort study?
to avoid reverse causality
What are advantages of the prospective cohort study design?
- Outcome bias is prevented. Measurement of exposure is not biased by knowledge of outcome status.
- Multiple outcomes/risk factors in single study
- Rare EXPOSURES (do not read: diseases) can be studied
What are disadvantages of the prospective cohort study design?
- People may have changed their behaviours over the intervening years (misclassification)
- Not good rare diseases
- follow-up time long enough?
- Costs
- Ensuring people who started to stay till the end: who is lost to follow up?
- Confounding in exposure/non-exposed? (e..g who are those who regularly use and those who do not regularly use their phones?)
What are limitations to the retrospective/historical cohort design?
- requires good records of past exposure
- generally limited to studying mortality or cancer outcomes, given the lack of universal records for other non-fatal endpoints
Shape case-control?
disease/no disease > exposure
Adv case-control study?
- Quicker
- Good for rare outcomes
- often less burden to participants
- cheaper
What is a disadvantage of selecting hospital controls in case-control group?
Drawback: their risk factor distributions may not reflect that of the source population from which the cases emerged
Disadv case-control study?
- Selection bias in case (survivor effect) and control groups (risk factors not like source population)
- Recall bias
- not good rare exposures
- Reverse causation
- Not able to calculate RR - you cannot calculate the incidence
- Confounding
Cross-sectional study: characteristics and major issue?
- may be conducted to gather information about any aspects of health and lifestyle
- participants should be recruited without knowledge of their exposure/disease status
- Reverse causality (establishing temporality) = major issue
What type of data can you use for an ecological study?
- Can only use generalized, already existing data
Why do we need descriptive data?
● To evaluate the occurrence of health behaviours and health conditions (disease)
- Time trends
- Specific population subgroups
● To provide a basis for planning and evaluation of interventions
● Descriptive data can be used for further analytic studies
Name three types of descriptive studies
- Case reports / series
- Prevalence studies / surveys
- Health registries / routinely collected data
What is meant with case reports/series?
- Detailed report of symptoms, diagnosis, treatment, and follow-up of individual patient(s)
- Important for the early identification of health problems (e.g AIDS)
- Can generate hypotheses about potential causes
- Case report: on 1 patient
- Case series: on more patients
What is meant with a prevalence study/survey?
- Presence of a condition at a particular point in time (point prevalence) or period (period prevalence) in a defined population
● Total number of people with certain disease
● Distribution of risk factor (e.g. smoking, blood pressure, obesity) or other characteristic (e.g. seat belt use)
By what factors is prevalence influenced?
● Incidence
● Disease duration
● Other factors, e.g. likelihood of diagnosis
Why are prevalence studies important?
- To assess the burden of disease in a population and to assess the need for health services
- To compare the prevalence of disease across populations
- To examine time trends in disease prevalence or severity
What happens when changes in diagnostic criteria for diseases and changes in coding procedures for morbidity take place?
Can lead to ‘artificial’ changes in prevalence (and incidence) figures.
Name four examples of health registries/routinely collected data
- Mortality records
- Hospitalisation records
- GP records
- Cancer screening data
- Birth records
- Child care data
- Infectious disease surveillance
- etc.
Shape of a cross-sectional study?
Defined population > Gather data on exposure + disease simultaneously > four groups (all needed): + exp + disease, + exp - disease, - exp - disease, - exp + disease
Shape of a prospective cohort study?
Defined population > choice/circumstance > 2 groups: exposed vs non-exposed > 4 groups (2 each): disease and no disease.
E.g. Choice: smoking.
2 group: smoking, non-smoking
4 groups: disease, no disease.
Shape of case-control study?
Retrospective
Defined population >looking back at cases and controls > each having 2 exposed and non-exposed groups
RCT shape?
Defined population > random allocation > 2 groups: exposed vs non-exposed > 4x disease vs not diseased
Which study designs are observational: analytical/descriptive, which are experimental?
Descriptive, Analytical:
Cohort
Case-control
Cross-sectional
Descriptive, observational:
With no comparison group
Experimental:
RCT, non-RCT
What is a cross-over design also called?
Two-period design or AB/BA design
What are conditions for a cross-over study? What is the period called between treatment and control period?
- Can only be used if the exposure has no long lasting effect
- Can only be used for outcomes that are reversible and that can change within a short time period
‘wash-out’ period
What is a community trial? By whom is it executed? What is usually the purpose?
- Trial in the population, often focused on primary disease prevention
- Unit of randomization are communities (e.g. towns, schools), not individuals
-> Intervention is provided by e.g. GPs, community health centers, local outpatient facilities.
- Suitable design for testing lifestyle interventions that cannot be allocated to individuals
Quasi-experimental study = ? Why use it?
studies that aim to evaluate interventions but that do not use randomization.
-> are less expensive and require fewer resources
Disadv RCT?
- Relatively high costs
- Rare disease outcome cannot be studied because of limited duration
- Cannot be used for non-modifiable risk factors (e.g. genes, birth weight)
- May be unethical, e.g. virus infection, smoking, sexual behavior
- Wrong ‘time window’ of exposure
- Not the full range of exposure can be studied
What study design to use when studying non-modifiable risk factors?
Cohort study
Purpose cohort study? Example?
- Evaluate the occurrence of disease in a carefully defined group of people (monitoring)
- To investigate the causes of disease and to establish links between risk factors and health outcomes (etiology)
EPIC (europe)
What do you do in a historical cohort study?
- Identify a population and observe events as they occur
- Retrospectively determine their exposure status from historical records
Drawbacks Ecological study?
- Populations differ in many ways other than the characteristic of interest
- Results may not apply to the individual.
Cross-sectional study: adv vs disadv?
Adv:
- relatively simple
- cheap
- quick
- minor ethical issues
Disadv:
- not rare disease
- hard to establish temporality (which came first: exp-disease ?)
Nested case-control study = ?
Cases of a disease that occur in a cohort are identified and, for each, a specified number of matched controls is selected from among those in the cohort who have not developed the disease by the time of disease occurrence in the case
