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29 Cards in this Set
- Front
- Back
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Experimental Studies
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Randomized and non-randomized
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Observational Studies Without comparison group
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Case report and case series
Cross-sectional/prevalence surveys |
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Observational Studies With comparison group
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Cross-sectional/prevalence surveys
Case-Control Cohort studies |
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Observational Studies
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Simple descriptive account of interesting characteristics observed in group of patients
Generally: Patients seen over a relatively short period Do not include control subjects Not planned studies Do not involve any research hypothesis Important role as a precursor to other studies |
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Cohort Studies
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Prospective or Historical/Retrospective
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Cross-Sectional Studies can be
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Surveys
Epidemiological studies Prevalence studies |
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Cross-Sectional Studies address
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Analyze data collected on a group of subjects at one time rather than over a period of time
Subjects selected and information gathered in a short period of time May be done to answer questions raised by a case-series study or may be done without a previous descriptive study |
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Case-Control Studies Begin with
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the presence or absence of an outcome and then try to look backwards in time to detect possible causes or risk factors that may have been suggested in a case series report.
Cases: individuals selected on the basis of some disease or outcome Controls: individuals without the disease or outcome History or previous events of both cases and controls are analyzed in an attempt to identify a characteristic or risk factor present in the cases’ histories but not in the controls’ histories Retrospective studies Matching |
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Cohort Studies
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Cohort: A group pf people who have something in common and who remain part of a group over an extended period of time
Subjects are selected by some defining characteristic (s) suspected of being a precursor to or risk for a disease or health effect Examine what happens to the disease over time as well as the causes of the disease. Also called prospective studies |
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Example of cohort study
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Framingham study of cardiovascular disease
Began 1948 to investigate factors associated with atherosclerotic and hypertensive cardiovascular disease Subjects: more than 6000 citizens of Framingham, masschussets Long term study, follow-up interviews and exams every two years Historical cohort study |
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Experimental studies in medicine that involve humans are called clinical trials
Controlled trials |
studies in which the experimental drug or procedure is compared with another drug or procedure, sometimes a placebo, sometimes a previously accepted treatment
Studies with controls are much more likely to detect that the observed difference is due to the intervention. |
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Meta-analyses are one type
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Use published information from other studies and combine the results to reach an overall conclusion
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Meta-analyses Especially useful when
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studies have small sample sizes or come to different conclusions
Include quantitative assessment & summary of findings |
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Threats to internal validity
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Bias
Confounding (confounding bias) Chance |
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Bias
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systematic error or variation in how patients selected, outcomes measured, data analyzed
Selection (volunteer) Measurement (information, recall) |
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Confounding (confounding bias)
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– factor associated with intervention that independently causes/associated with outcome that influences relationship of intervention to outcome
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External Validity (or generalizability)
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Doesn’t affect internal validity or quality
Applicability to a particular patient or population |
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Causality
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Biologic Plausibility
Statistically significant, strong association Temporal relationship - necessary Repeatability/Reliability |
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Interaction or Effect modification
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The association between exposure and disease differ in strata of the population
Example: Tetracycline discolours teeth in children, but not in adults |
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Confounding is a bias that you hope to prevent or control
Something to avoid Interaction or effect modification is just a |
more detailed description of the effect itself
Something to report |
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Observational Studies: Cross-Sectional Studies advantages
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Appropriate for:
Prevalence studies Evaluating diagnostic procedures Quick to complete Relatively inexpensive |
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Observational Studies: Cross-Sectional Studies disadvantages
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Can’t establish cause (temporal relationship)
Can’t establish risk since incidence not measured Surveys Obtaining sufficiently large response rates Those who agree to participate may not be representative of the population (selection bias) |
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Observational Studies: Case-Control studies advantages
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Appropriate for:
Studying rare diseases or outcomes/events Events that develop over a long time Investigating preliminary hypothesis Quickest Least expensive |
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Observational Studies: Case-Control studies disadvantages
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Opportunities for multiple biases
Selection bias (appropriate controls and cases) Recall bias (memory errors) Information bias (quality of existing data) Compromise between what is available and what one wants to study Can’t establish cause as can’t measure incidence |
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Observational Studies: Cohort Studies Prospective or Historical advantages
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Appropriate for studying
Course of a disease or Prognosis Risk factors Rare exposures Organized data collection Reliable (no recall bias) Clear time order and true measurement of incidence |
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Observational Studies: Cohort Studies Prospective or Historical disadvantages
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Biases – mostly because not randomized
Selection, measurement, and confounding Loss to follow-up Time-consuming Expensive |
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Observational Studies: Cohort Studies Prospective or Historical
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Longitudinal and follow a group over a period of time
Causation generally cannot be proved as they are observational and do not involve intervention However, correct time sequence thus provide strong evidence for possible cause and effect, example smoking and lung cancer controversy |
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Experimental Studies: Randomized, controlled trials (RCT’s) and non-random advantages
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Gold standard for best internal validity
Best for establishing cause and evaluating therapy Subject to least number of biases If randomized (assigned by chance) decreases selection bias If blinded decreases measurement bias (single, double, triple) If randomized well, decreases confounding bias between members of intervention group versus control group |
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Experimental Studies: Randomized, controlled trials (RCT’s) and non-random disadvantages
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Loss to follow-up is biggest threat to internal validity
Aside from intervention, were groups treated equally? Time-consuming and long duration Expensive Volunteer bias Intention to treat analysis Ethical constraints/dilemmas External validity (generalizability) |
