Epidemiology (7) - Observational Studies
Case reports & series
Accounts of single occurrence of noteworthy health-related incident or collection of such events (e.g., case-series)
Types of analytical observational studies
Cross-sectional and ecological Case-control Cohort
Disadvantages of case reports
Data availability and accuracy is a challenge, esp. if retrospective Not based on systematic study Cases may not be generalizable Causes or associations may have other explanations
Key measure of cross-sectional study
Disease prevalence in those with and without exposures or at different exposure levels are compared
Types of descriptive observational studies
Disease surveillance Case report Case series
Disadvantages of ecological studies
Ecological fallacy: Applying exposure and outcome findings from the group level to the individual level Cannot establish a temporal relationship between exposure and outcome Confounding factors may not be known
Advantages of of case-control studies
Efficient for rare diseases Efficient for diseases with long induction and latent periods because you are starting with the outcome (the case) Can evaluate multiple exposures in relation to a disease Relatively quick and inexpensive
Cross-sectional studies
Exposure status and disease status of an individual are measured at one point in time
Cohort studies
First focus on an exposure: "Exposed" and "unexposed" are selected by the investigators to be followed longitudinally over time to observe difference in the incidence of the outcome (so it is exposure then determine the outcome)
Analytical
Have a comparison group (cross-sectional weakest in this respect—comparison gr. may be unstated and assumed as the larger population); sometimes called "natural experiments"
Cross-sectional studies are useful for
Health planning Hypothesis generating, not etiology (cause) proving
Advantages of case reports
Help identify new clinical issues and may lead to new hypotheses, ex.: Drug side effects and potential new uses (adverse and beneficial) Rare manifestations of a disease New characteristics of diseases Propose (not prove) new etiologies of diseases, e.g. Zika and Microcephaly Educational (maybe)
Disadvantages of cohort studies
Inefficient for evaluation of rare diseases If outcome has long latent period, study can take a long time Generally more expensive If retrospective, requires availability of records to minimize recall (memory) weaknesses Validity of results can be seriously affected by losses to follow-up
Disadvantages of cross-sectional studies
Lack of temporal sequence of exposure preceding disease, thus only describes an association Tends to include cases with long duration, which may have different characteristics and risk factors than series of incident cases Potential misclassification of disease status if disease has exacerbations and remissions (e.g. asthma, multiple sclerosis, lupus) or if disease is being treated (e.g. hypertension)
When to initiate case-control
Little is known about the disease Exposure data are difficult or expensive to obtain Rare disease (but you can identify cases of the disease, ex. Ebola) Disease with long induction and latent period Dynamic underlying population (too hard to track as required in cohort or RCT)
Relative risk
Measure of association used for deriving a causal inference
Pros/cons of prospective
More expensive, time consuming Not efficient for diseases with long latent periods Better exposure and confounder data—collect real-time as part of study Less vulnerable to recall bias
Advantages of cohort studies
Useful design when exposure is rare Examine multiple effects of single exposure (multiple outcomes) If prospective, minimize bias in ascertainment of exposure because you eliminate recall (memory) weaknesses or bias Can examine temporal relationship between exposure and disease (good for establishing causality) Allows direct measurement of incidence of disease in exposed and unexposed, thus calculation of Relative Risk
Matching
Without randomization, cases and controls may differ in characteristics Individual matching (pairwise or multiple)
Selection of case-control studies
Should lead to accurate classification of diseased and non-diseased individuals Homogeneous disease entity by strict diagnostic criteria, e.g. distinguishing cancer of the corpus uteri (body of uterus) from cancer of the cervix (neck of uterus) Applied uniformly
Observational studies
Used to study a wider range of exposures than experimental studies "Natural" experiments Mitigate many issues which are not feasible in experimental studies
Calculating odds ratio
ratio of the probability of the occurrence of an event to that of nonoccurrence.
Ecological studies
relate the frequency with which some characteristic and some outcome of interest occur in a group, perhaps in the same geographic area. Provide quick inexpensive answers to group phenomena
Prospective cohort study
the study population is identified at the beginning of the study and then followed through time.
Retrospective cohort study
the study population is identified in the past and then, typically through records, exposure and disease status are determined.
case-control study
A method of sampling a population in which cases of disease are first identified and enrolled, then a sample of the population that produced the cases (e.g., a comparable population) is identified and enrolled as the control group Exposures are then determined for individuals in each group
Disadvantages of case-control studies
Can investigate only one disease outcome (this is the Case) Cannot directly compute incidence rates of disease in exposed and unexposed (Odds ratio ≠ Relative Risk) Temporal relationship between exposure and disease may be difficult to establish Vulnerable to bias because retrospective (recall bias)
Pros/cons of retrospective
Cheaper, faster Efficient with diseases with long latent period Exposure data may be inadequate Potential for biased recall (remembering)
Advantages of cross-sectional studies
Often have reasonably good generalizability if sample size is large enough Many outcomes and risk factors can be assessed simultaneously Often conducted in a relatively short period of time Less costly than cohort and case-control studies
_____ study is used for diseases with a long latent period such as cancer
Retrospective
Ecological studies is a subset of _______ study
cross-sectional
If relative risk = 1
exposure is NOT associated disease
If relative risk < 1
exposure is associated with a decreased risk of disease (i.e. is protective)
If relative risk > 1
exposure is associated with an increased risk of disease
Descriptive
focus on person, place, time
Ecological studies look at
group characteristics
Calculating relative risk
incidence in exposed / incidence in unexposed
Selection of Controls
must be identified independently of exposure status Controls are a sample of the population that gave rise to the cases (member of control group who gets the disease "would" otherwise end up as a case in the study)
Case reports generally report
new or unique findings; Undescribed disease Adverse reaction Potential link between disease and exposure Treatment outcome
Key measure of case-control study
odds ratio