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Designing Cross-Sectional & Cohort Studies in Research

Learn about different study designs in research, such as observational, experimental, cross-sectional, and cohort studies, their advantages and disadvantages, and how to apply them to investigate relationships between variables.

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Designing Cross-Sectional & Cohort Studies in Research

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  1. Research Design:Choosing the Study Design Kimberly R. Barber Director of Research McLaren Regional Medical Center

  2. Design to Employ • Dependent on the hypothesis posed. • Is your intent to observe, associate factors, or show cause and effect? • Are exposure or outcome factors common or rare? • Are your resources many or constrained?

  3. Types of Study Designs • Observational Designs • Descriptive • Analytic • Cross-sectional, Case-control, Cohort • Experimental Designs • Quasi experimental • Non randomized or non control trial • True experimental • Randomized Controlled Trial

  4. Descriptive Studies • No assignment of exposure or risk factor. • Intent is to observe and record. • Record events or activities. • Single event or case = Case Report. • Several events or cases = Case Series.

  5. Cross-Sectional Studies • Measurement of risk and outcome at the same time. Risk factor Outcome

  6. Cross-Sectional Design • The only study capable of calculating prevalence. • Proportion of the population with the outcome at any point in time.

  7. Cross-Sectional Studies (cont) • Most useful if exposure continues right up to time that outcome is recognized. • Often conducted as screening and classification preamble to prospective studies.

  8. Application Exercise: Cross-Sectional Study • Design a cross-sectional study that examines the relationship between dietary sodium and hypertension in middle-aged men. • What research question can we answer?

  9. Cross-Sectional StudiesAdvantages • Cheap and quick studies. • Data is frequently available through current records or statistics. • Ideal for generating new hypothesis.

  10. Cross-Sectional StudiesDisadvantages The importance of the relationship between the cause and the effect cannot be determined. • Temporal weakness: • Cannot determine if cause preceded the effect or the effect was responsible for the cause. • The rules of contributory cause cannot be fulfilled.

  11. Case-Control Studies (Retrospective) • Case/Control Studies. • Group subjects according to the presence or absence of the outcome. • Review past histories of the subjects for the occurrence of suspected risk factors. ? ? Risk Disease

  12. Case / Control Studies (cont) • Case / Control studies have two main purposes: • Descriptive • Describe the risk factor profile for an outcome. • Analytic • Analyze associations between outcome and risk factors.

  13. Application Exercise:Case / Control Study • Describe a case/control study on the relationship between childhood obesity, smoking history, and occurrence of hypertension in middle-aged men. • What research question can we answer?

  14. Case / Control StudiesAdvantages • Are good initial explanatory studies. • Investigators can explore multiple risk factors simultaneously for one outcome. • Are efficient, relatively cheap, and quick. • Data available through chart review.

  15. Case / Control Studies Advantages (cont) • Well suited for rare diseases. • Since the study begins with subjects who already have the outcome, it is easier to accumulate enough subjects for significant results.

  16. Case / Control StudiesDisadvantages • Data Quality • Data with inadequate detail, questionable reliability, or use a different standard to judge disease severity. • Recall bias • Subjects who have unpleasant experiences may recall past differently than control subjects.

  17. Retrospective StudiesDisadvantages (cont) • Sampling bias • Sample usually not representative of all subjects who could be included. • Other • Capable of studying only one outcome at a time • Cannot calculate prevalence or incidence. • Subject to confounding factors. • Cannot prove contributory cause.

  18. Cohort Studies(Prospective) • Subjects identified according to the presence or absence of the risk factor. • Followed over time until the outcome occurs or becomes evident.

  19. Cohort Studies (cont) • Subjects with and without the suspected risk factor are followed for the development of the outcome. • The frequency of the outcome is compared between the two groups. Disease R i s k ? ?

  20. Cohort Studies (cont) • Cohort studies have two main purposes: • Descriptive • Describe the incidence of outcome over time. • Analytic • Analyze associations between risk factors and outcome.

  21. Historical Cohort Studies(Non concurrent) • The following all happen in the past: • Assembly of the cohort according to the presence of absence of risk factors. • Baseline measurements • Follow-up and outcomes ? ? Exposure Disease

  22. Historical Cohort (cont) • Are not retrospective studies. • Subjects are grouped according to the presence or absence of risk factors (past or present). • Subjects in retrospective studies are grouped according to the presence or absence of the outcome.

  23. Cohort StudiesAdvantages • More powerful design for defining incidence. • Powerful design for associating the cause with the effect. • Can suggest that the cause precedes the effect. • Data can be collected in a comprehensible and uniform fashion. • Exception: historical cohort with same measurement weaknesses as the case/control.

  24. Cohort StudiesAdvantages (cont) • No recall bias. • Cohort designs can examine many outcomes for potential risk factors under investigation.

  25. Cohort StudiesDisadvantages • Expensive in time, money, and number of patients necessary to demonstrate significant differences between groups. • Less likely than retrospective studies to uncover new risk factors. • Also subject to confounding.

  26. Cohort StudiesDisadvantages (cont) • Loss of valuable information due to patient attrition. • Patients may change their behaviors or risk factors after the initial grouping of subjects resulting in misclassification and study error.

  27. Clinical Trial • Experimental study • Unique features: • Intervention in the subject’s care for the purposes of the study. • Randomization of subjects. • Control group comparison. • Placebo or treatment

  28. Clinical Trial Design Outcome Randomized X X Intervention/ Effect

  29. Clinical Trial (cont) • Randomization • Subjects are randomly assigned to control or experimental group. • Groups are similar in every way except for the intervention under study. • Each subject has equal probability of being placed in either group.

  30. Clinical TrialsAdvantages • Subject to the fewest methodological biases of all study designs. • Most powerful study designed to show contributory cause.

  31. Clinical TrialsDisadvantages • Is the most expensive study design in terms of money, time, and number of patients. • Issues of patient attrition and compliance may invalidate the results. • Can be problematic for ethical reasons. • Use of placebo • Harm outweighing benefits • Zero tolerance for some exposures

  32. Choosing the Specific Design • Study design is highly dependent on the type of analysis. (3) • Type of analysis is dependent on the hypothesis posed. (2) • The hypothesis is dependent on the intent of your research. (1)

  33. 1. Research Study Intent • Know the problem. • Determine what you want to conclude. • Formulate the question.

  34. Examples of Intent • I intend to show that aspirin resistance is associated with the severity of heart disease. • I will compare levels of aspirin resistance among patients with differing severity of heart disease. • I intend to show that breast feeding is protective against allergies developing in infants. • I will compare rates of allergies among women who breast feed and those who do not.

  35. 2. Research Hypothesis • Know the question you want answered. • Restate the question into terms of Ho and Ha. • Think about corresponding analysis.

  36. Examples of Hypotheses • Is aspirin resistance associated with heart disease? • Aspirin resistance increases the risk of heart attack. • Is breast feeding associated with allergies? • Breast feeding decreases the risk of allergies in babies.

  37. 3. Statistical Plan of Analysis • Correlation? • Comparison? • Association? • Difference?

  38. Examples of Analysis • The level of aspirin resistance is compared between those with heart attack and those without. • Differences in resistance scores between two groups (Student T-test) • The rate of infant allergies is compared among women who breast feed and those who do not. • Relative risk association (Chi square analysis)

  39. Choose Appropriate Design • Cross-sectional. • Case-control. • Cohort. • Clinical Trial.

  40. Apply the Best Design • Think about the measures to be used. • Know the analysis required. • Rethink desired conclusions.

  41. Summary • Study Design key to determining cause or just association. • Each design has unique strengths & weaknesses. • The design drives the conclusions.

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