Epidemiology (Lecture 17) – Cross sectional studies – Flashcards
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• Information is gathered from a sample of the population about present status of: - Health Behaviors - Diseases - Risk Factors • Information is gathered at only one point in time
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General cross sectional design
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Cutting through population at one point in time E.g., at one section of bread that you cut, there is a certain distribution of nuts and raisins Might get different distributions at different sections and times.
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Analogy of cross sectional
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• Purpose of descriptive study is to describe the population of interest • Used to calculate prevalence of health behavior, risk factor or disease. E..g, youth risk behavioral surveillance system
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Purpose of descriptive cross sectional studies
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Slide 9
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Analytic study design tree
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Take sample of population Collect measurements Ask about exposures to risk factors Ask about health conditions of interest
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Cross sectional design figure
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• An analytic study tells us how two things are related or associated • A descriptive cross-sectional study could be made into an analytic cross-sectional study - Just need to consider two variables at once • Could use data, such as YRBSS data presented, to make comparisons - Is drug use associated with violence? - Is cigarette smoking associated with drug use?
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Analytic vs. Descriptive cross sectional studies
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Measuring exposure and outcome at the SAME point in time
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Timing of cross sectional study
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• Calculated just like a Relative Risk / Risk Ratio • Used in cross-sectional studies, which measure prevalence • Uses prevalence instead of incidence (risk) to calculate ratio Look at slide 13 for example
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Prevalence ratio
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Slides 15 & 16 Same formula as RR
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Example setting up prevalence ratio
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What came first If you measure exposure and disease at the same time, how do you know what came first
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Big question with cross sectional studies
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• Can be done quickly because do not follow over time • Relatively low cost (depending on size) • Can calculate odds ratio to test strength of association • If randomly sample a population, can estimate prevalence of risk factors and conditions, and calculate prevalence ratio • Can assess many risk factors and many outcomes at same time
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Advantages of Cross sectional design
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• Can be done quickly because do not follow over time • Relatively low cost (depending on size) • Can calculate odds ratio to test strength of association • Do not necessarily know if risk factor preceded disease • Cannot assess rare outcomes (unless very large) • Cannot estimate incidence of disease
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Similarities between C-S and C-C design
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• Do not necessarily know if risk factor preceded disease • Cannot assess rare outcomes (unless very large) • Cannot estimate incidence of disease - because only measure at one point in time
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Disadvantages of Cross sectional study
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Experimental studies the investigator chooses who gets exposure Observational studies the investigator observes who has the exposure
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Experimental vs. observational studies
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- Temporality (exposure before outcome) - Confounding - Speed/cost of completion - Examination of multiple risk factors - Examination of multiple outcomes - Examination of risk factors that are dangerous and therefore unethical to manipulate - Good for rare outcomes
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How do different research designs differ?
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• RCT and Cohort Study - Exposure: Present - Outcome: Future • Case-Control Study - Outcome: Present - Exposure: Past • Cross-Sectional Study - Exposure: Present - Outcome: Present
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Timing of study designs
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- Establishes temporality - All potential confounding controlled! - Can follow for multiple outcomes
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Strengths of RCT
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- Expensive and slow - Not suitable for: • dangerous exposures that cannot be manipulated for ethical reasons (e.g. drunk driving) • innate characteristics that cannot be assigned (e.g., personality traits - Typically only one or a few exposures can be studied - Volunteer subjects may differ from general population
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Weaknesses of RCT
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- Establishes temporality - Can follow for multiple outcomes - Good for dangerous exposures that cannot be randomly assigned for ethical reasons
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Cohort study strengths
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- Expensive and slow - All associations potentially confounded - Not good for rare outcomes
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Cohort study weaknesses
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- Quick results, less expensive - Good for rare outcomes - Can study multiple exposures
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Strengths of case control study
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- Temporality can be difficult to establish - All associations potentially confounded - Cannot estimate incidence of disease - Two selection processes that may produce bias *one for cases and one for controls - Recall bias may be a concern
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Weaknesses of case control study
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- Quick results, less expensive - Can study multiple exposures AND multiple outcomes - If randomly sample whole population, can estimate prevalence of risk factors AND conditions
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Strengths of cross sectional study
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- Temporality can be difficult to establish - Cannot estimate incidence of disease - All associations potentially confounded - Recall bias may be a concern
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Weaknesses of cross sectional study
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Slide 34 VERY HELPFUL
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Overall table of strengths of each study design
