Lecture 20-21: Case Control Studies Flashcards Preview

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Flashcards in Lecture 20-21: Case Control Studies Deck (16)
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1
Q

List the order of an Observational study design from least evidence to largest evidence

A

Observational:

  1. Case Reports/Series
  2. Ecological
  3. Cross-Sectional
  4. Case-Control
  5. Cohort (Highest evidence)

Note: Observational studies cannot prove causation the way interventional studies can, but they are still important.

2
Q

Describe the case-control study design

A
  • Case-Control studies are OBSERVATIONAL studies allowing researcher to be a passive observer of natural events occurring in individuals with the DISEASE/CONDITION OF INTEREST (CASES) who are compared with people who DO NOT HAVE THE CONDITION OF INTEREST (CONTROLS)
  • The Control group supplies information about the expected baseline risk-factor profile in the population from which the Cases are drawn
    • GROUP-ASSIGNMENTS ARE DIVIDED/BROKEN-UP/BASED ON DISEASE STATUS
  • USEFUL WHEN STUDYING A RARE DISEASE OR INVESTIGATING AN OUTBREAK (so if it only occurs amont 18 people every 10 years, gonna have to go to the source of an outbreak for studying)
3
Q

What is common about all observational studies?

A

Not Randomized! They are divided based on whether or not they are diseased/exposed or not.

So Population breaks down into whether or not they’re diseased, both results of which break down into whether or not they were exposed or not.

Case Control Design is Useful when Studying A Rare Disease. Commonly generates an ODDS OF EXPOSURE for each, then an ODDS RATIO (OR) as a measure of association. As far as finding a control group, we can “Randomly Select” people who fall into control groups while remaining blind to their possible exposure status. That right there is Not the same thing as Randomized and is probably the only time the word Random is used in an observational study.

4
Q

What is the 2x2 table trying to determine in an observational study design?

A

Case control study is trying to find out: Out of everybody in the diseased category (A+C), what percentage/proportion of them differs from those in the undiseased category (B+D) were similarly exposed.

5
Q

Give the characteristics of reasons to select case-control studies

A
  • Unable to force group allocation (‘randomize’)
  • Unethical / Not feasible
  • Limited resources
  • Time / Money / Subjects
  • The DISEASE OF INTEREST is rare in occurrence and little is known about its associations/causes
  • Prospective exposure data, derived from prospective Cohort study, is difficult/expensive to obtain and/or very time inappropriate
  • Case-Control studies are customarily conducted in a Retrospective fashion
6
Q

What are the Strengths of case control study designs compared to other designs?

A
  • Good for assessing Multiple Exposures of ONE outcome
  • Useful when DISEASES are rare
  • Useful in determining Associations (NOT Causation)
  • Less expensive (money/time) than Interventional trials and prospective Cohort studies
  • Useful when ethical issues limit Interventional studies
  • Useful when disease has a long induction/latent period
  • Weaknesses of Case-Control studies may be the opposite of these general points listed above
7
Q

Describe the factors that help define proper selection of cases

A
  • Defined by the investigator using accurate, medically-reliable, and efficient data sources
  • Applied to all study participants
    • OBJECTIVELY, CONSISTENTLY, ACCURATELY, AND WITH VALIDITY – Clinically-supportable/definable criteria are best!
    • From published, professionally-recognized and accepted diagnostic criteria and/or from multiple sources of data
  • Labeling patients CORRECTLY is ideal, but always-present is the risk of ‘misclassifying’ subjects (into wrong group)
8
Q

Recall counterfactual theory in regards to evaluating an association

A
  • All else being equal (in the same group), the outcome if something DIDN’T occur
  • Counterfactual outcome for Smokers ESTIMATED by Non-Smokers (surrogate representative group)
  • Requires assumption of EXCHANGEABILITY
  • EXCHANGEABILITY = COMPARABILITY with respect to all other determinants of outcome
9
Q

Describe the control selection for case-control studies

A
  • Most difficult part is control selection
  • Goal: to assess for the presence of an association between exposure & known condition of interest by selecting non-disease individuals from the sample population which produced the Cases
  • Expectation is the Controls represent the baseline risk of exposure in the general or reference population
  • The way the controls are selected is a MAJOR DETERMINANT in whether any conclusion is valid
  • INTERNAL VALIDITY
  • SELECTION BIAS
10
Q

What is the proper way to select controls for a case-control study?

A
  • Make the groups as close as possible EXCEPT the presence of the disease (outcome) of interest
  • If exposure truly has no effect, then odds will be exactly the same for both groups and OR will be 1.0 (no difference)
  • *Controls must be selected irrespective of exposure status!
11
Q

What are the 3 main sources that a control group can come from?

A

(Can come from multiple sources)

  1. Population’ (State/Community/Neighborhood)
    - Can be obtained via numerous avenues, even RANDOMLY!
  2. Institutional/Organizational/Provider
    - Illness(es) of Controls should be unrelated to exposure(s) being studied
  3. Spouse/Relatives/Friends
    - Genetic, Environmental, Socio-Economic, etc… similarities

(Unofficial) 4. Outbreak-sources of Controls:
- Participated in same event (e.g., picnic / convention)

12
Q

What is the somewhat paradoxical function about exposure in a case-control study

A
  • AN INDIVIDUAL CAN ACTUALLY FUNCTION AS BOTH AN EXPOSED INDIVIDUAL AND AN UNEXPOSED INDIVIDUAL IN THE SAME STUDY
  • Can be associated with an outbreak investigation with Multiple exposures, OR…
  • In an a situation of a brief (acute) change in risk of the outcome in interest (hazard period):
    • Called a ‘CASE-CROSSOVER’ design: Subjects are their own Controls during the other times they don’t have the acute change in risk. The only Case-Control design able to adequately attempt to address issue of “temporality”
13
Q

Describe Nested Case Control Studies

A
  • These are Case-Control studies conducted after, or out of, a prospective Cohort study
  • Subjects in Cohort study ultimately developing disease are defined as Cases for the subsequent Case-Control study
    • Diseased used in a new (different) study. Used to evaluate other exposures
14
Q

Describe Control Sampling for Nested Control Studies

A
  • Selection of Controls used for Nested Case-Control studies or when ‘sampling’ necessary from Cohort (source population):
  • SURVIVOR sampling
    • Sample of non-diseased individuals (survivors) at end of study period
  • BASE sampling
    • Sample of non-diseased individuals at start of study period
  • RISK-SET sampling
  • Sample of non-diseased individuals during study period at same time when Case was diagnosed
15
Q

What are common biases in case control studies?

A
  • SELECTION BIAS is related to the way subjects are chosen for study (usually more of a concern for Control selection)
  • Less concern during Case-Crossover study designs
  • RECALL BIAS is related to the amount/specificity that Cases or Controls recall past events DIFFERENTLY
  • More-commonly Cases more likely to RECALL past exposures and levels of exposure (or their timing)
16
Q

How are cases matched to controls?

A
  • In some studies, Cases are matched to Controls (in a 1:1, or higher, ratio)
  • INDIVIDUAL matching:
    • Matches individuals based on specific patient-based characteristics. Useful for controlling confounding characteristics
  • GROUP matching:
    • Proportion of Cases & proportion of Controls with identical characteristics are matched (Ex: 41% of Cases are male, so 41% of Controls are male). Requires Cases be selected first
  • DON’T MATCH ON ANYTHING THAT MIGHT BE A RISK FACTOR