Epi511 Set 2 Case Studies

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Randomized Controlled Clinical Trial (RCT):

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prospective, analytical, experimental study using primary data generated in the clinical environment. Individuals similar at the beginning are randomly allocated to two or more treatment groups and the outcomes the groups are compared after sufficient follow-up time

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Randomized Cross-Over Clinical Trial:

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A prospective, analytical, experimental study using primary data generated in the clinical environment. Individuals with a chronic condition are randomly allocated to one of two treatment groups, and, after a sufficient treatment period and often a washout period, are switched to the other treatment for the same period

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Observational Studies:

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Cohort(Incidence, Longitudinal), case-control, Ecologic (Aggregate) Study, Cross-Sectional (Prevalence Study) Study, Case Series, Case Report

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Cohort (Incidence, Longitudinal Study) Study:

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prospective, analytical, observational study, based on data, usually primary, from a follow-up period of a group in which some have had, have or will have the exposure of interest, to determine the association between that exposure and an outcome

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Case-Control Study

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A retrospective, analytical, observational study often based on secondary data in which the proportion of cases with a potential risk factor are compared to the proportion of controls (individuals without the disease) with the same risk factor. The common association measure for a case-control study is the odds ratio.

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Ecologic (Aggregate) Study:

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An observational analytical study based on aggregated secondary data. Aggregate data on risk factors and disease prevalence from different population groups is compared to identify associations.

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Cross-Sectional (Prevalence Study) Study:

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descriptive study of the relationship between diseases and other factors at one point in time (usually) in a defined population.

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Case Series:

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descriptive, observational study of a series of cases, typically describing the manifestations, clinical course, and prognosis of a condition

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Case Report:

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Anecdotal evidence. A description of a single case, typically describing the manifestations, clinical course, and prognosis of that case.

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Descriptive Study:

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objective of a descriptive study is to describe the distribution of variables in a group. Statistics serve only to describe the precision of those measurements or to make statistical inferences about the values in the population from which the sample was taken.

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Historical (Non-concurrent) Comparison:

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Comparison is of the same group or between groups at different times that are not experiencing the risk factor or the treatment at the same time. Historical comparison is often used to allow a group to serve as its own historical control or is done implicitly when a group is compared to expected standards of performance.

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Prospective Study (Data)

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Data collection and the events of interest occur after individuals are enrolled (e.g. clinical trials and cohort studies). This prospective collection enables the use of more solid, consistent criteria and avoids the potential biases of retrospective recall. Prospective studies are limited to those conditions that occur relatively frequently and to studies with relatively short follow-up periods so that sufficient numbers of eligible individuals can be enrolled and followed within a reasonable period.

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Retrospective Study (Data):

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All events of interest have already occurred and data are generated from historical records (secondary data) and from recall (which may result in the presence of significant recall bias).

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Confounding:

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: Confounding is the distortion of the effect of one risk factor by the presence of another. Confounding occurs when another risk factor for a disease is also associated with the risk factor being studied but acts separately

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External Validity (Generalizability):

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Truth beyond a study. A study is external valid if the study conclusions represent the truth for the population to which the results will be applied because both the study population and the reader’s population are similar enough in important characteristics.

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Analytic epidemiology

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attempts to specify in more detail the causes of a particular disease

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Factors Important in Study Design

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A. Specific, testable hypotheses - NOT a fishing expedition
B. Biases
1. Internal validity
2. External validity

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Other Epi designs

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Time series - test if incidence of disease changes in a population over time
Hybrid - often what is seen in practice
Can be efficient and match necessity
Can lead to bias and disaster
Meta-analysis
Combining results from a range of published studies
Established methodology, not just literature review

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What type of epi study to chose depends on:

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what is the research question/ objective
Time available for study
Resources available for the study
Common/rare disease or production problem
Type of outcome of interest
Quality of data from various sources
Often there are multiple approaches which will all work
Choosing an established design gives you a huge head start in design, analysis and eliminating biases

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Measures of Association, two main types

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Difference Measures (Two Independent Means, Two Independent Proportions, The Attributable Risk)
Ratio Measures (Relative Risk, Relative Prevalence, Odds Ratio)

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Attributable Risk (AR)

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The difference between 2 proportions
Quantifies the number of occurrences of a health outcome that is due to, or can be attributed to, the exposure or risk factor
Used to assess the impact of eliminating a risk factor

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Measures of Association:Ratio Measures

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Relative Risk (RR)
Relative Prevalence (RP)
Odds Ratio (OR)

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Association

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A statistical relationship between two or more variables

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Risk

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Probability conditional or unconditional of the occurrence of some event in time

Probability of an individual developing a disease or change in health status over a fixed time interval, conditional on the individual not dying during the same time period

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Relative Risk Ratio

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RR (Ratio of two risks; Risk Ratio; Relative Risk) CIE+ / CIE- = 28/17.4 = 1.6

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Interpretation of RR

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Smokers were 1.6 times as likely to develop CHD as were non-smokers

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Risk Difference

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Difference of two risks (Risk Difference)*
CIE+- CIE- = 28.0 – 17.4 = 10.6

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Odds Ratio

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OR as a measure of association between exposure & disease is used when data are collected in case-control study

OR can be obtained however, from a cohort as well as a case-control study and can be used instead of RR.

Odds are ratio of two probabilities
i.e. Probability that event occurs / 1-Probability that event does not occur

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When is the OR a good estimate of RR?

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In CCS, only OR can be calculated as measure of association
In Cohort study, either RR or OR is a valid measure of association
When a RR can be calculated from case control study?

*When exposure prevalence among studied cases in similar and nearly similar to that of disease subjects in the population from which cases are taken.
*Prevalence of exposure among studied controls is similar to that of non-diseased population from cases were drawn.
*Rare disease (CI < 0.1)

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Matched case-control study

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Matching: In a matched case-control study each case is matched to a control according to variables that are known to be related to disease risk i.e. age, sex, race
Data are analyzed in terms of case-control pairs rather than for individual subjects
Four types of case-control combinations are possible in regard to exposure history.

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OR in matched pairs

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Concordant pairs are ignored since they don’t contribute in calculation of effect estimate (i.e. OR)

Disconcordant pairs of cases and controls are used to calculate the matched OR.

Matched OR = Ratio of discordant pairs = b /c

i.e. # of pairs in which cases exposed / # of pairs in which controls were exposed

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Relative Risk and Cohort studies

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RR = I(e)/I(not) = a/(a+b)/c/(c+d)

RR = ID(e)/ID(o) = a/PY(1)/c/PY(2)

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RR = 1

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Incedence in exposed = incidence in unexposed = no association (null value of RR)

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RR > 1

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Incidence in exposed is GREATER

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RR < 1

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Incidence in exposed in LESS (POSSIBLY PROTECTIVE)