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Malimu introduction to study designs

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Malimu introduction to study designs

  1. 1. Introduction to Study designs Department of Epidemiology and Biostatistics, MUHAS.
  2. 2. Case-control Cohort Individuals Intervention Retrospective Prospective Descriptive Populations Analytical Observational Case-series Cross-sectional Correlational/ecological Clinical trials Epidemiological studies
  3. 3. Why do epidemiologic studies?  How big is the problem?  Are two factors related, (cause and effect)?
  4. 4. FELP Introduction to Descriptive studies
  5. 5. Case-control Cohort Individuals Intervention Retrospective Prospective Descriptive Populations Analytical Observational Case-series Cross-sectional Correlational/ecological Clinical trials Epidemiological studies
  6. 6. Learning Objectives When you have completed this session you will be able to:  Describe the differences between and descriptive and analytic studies  Describe the differences between a case report and a case series  Describe the characteristics of an ecological study  Describe a cross sectional study and explain its advantages and disadvantages  Explain the uses of the descriptive study types
  7. 7. Descriptive versus Analytical epidemiology Descriptive epidemiology: • generates idea(s) or hypothesis for associations between risk factor and illness Analytical epidemiology: • uses a comparison group to establish an association between risk factors and illness in the two groups
  8. 8. Descriptive Studies  The most frequent design strategy found in the epidemiologic literature  Used to describe the distribution of disease by time, place, person and assoc. factors  Describing these factors does not link them  However we can identify unusual distributions or correlations  Useful for Hypothesis generation and health planning
  9. 9. Using Descriptive Studies for Hypothesis Formulation  Person – “Who is getting the disease?” Age, race, sex  Place – “Where are the rates of disease highest and lowest?”  Time – “ When does the disease occur commonly or rarely?” and “ Is the frequency of the disease now different from the corresponding frequency in the past?”
  10. 10. Person Place Time Cases 0 5 10 15 20 25 1 2 3 4 5 6 7 8 9 10 0 200 400 600 800 1000 1200 0-4 '5-14 '15- 44 '45- 64 '64+ Age Group Descriptive Epidemiology Who? Where? When?
  11. 11. Categories of Descriptive Studies  Populations (correlational or ecological studies)  Individuals  Case reports  Case series  Cross-sectional surveys
  12. 12. Correlational or Ecological Studies Based on aggregate measures of exposure and outcome from several populations. The population is the unit of observation available for study. Exposures: - What percent of a population smokes? - What percent of 1-year old children are vaccinated against measles? - What percent of a population has piped water?
  13. 13. Correlational or Ecological Studies Outcomes: - What percentage of a population died from MI? - What percentage of children had measles last year? - What percentage of population had episodes of diarrhea?
  14. 14. Correlational or Ecological Studies Advantages -Easy to do -Use available data (“administrative” or other aggregate data) -Can be done in population with widely differing characteistics -Generate hypotheses for additional study
  15. 15. Correlational or Ecological Studies Disadvantages -Unable to examine data for individuals; data on exposure and data on outcome are collected independently -No assurance that persons with exposure (risk factor) of interest are the same ones with the outcome (disease) of interest -Association at the aggregate level may not reflect association at the individual level - the ecologic fallacy -Unable to adjust for potential confounding factors. -Poor correlation doesn’t mean no association
  16. 16. Descriptive Studies  Populations (correlational or ecological studies)  Individuals  Case reports  Case series  Cross-sectional surveys
  17. 17. Case reports The individual is the unit of observation available for study. Clinical case with “unusual” clinical picture May suggest an etiological association
  18. 18. Case series First case report may stimulate compilation of additional case reports….a case series (e.g. occurrence of Pneumocystis carinii among a group of young, homosexual men with no history of immune deficiency)
  19. 19. Case reports or Case series Advantages: Use available clinical data Detailed individual data Suggest need for investigation (hypothesis generation) Disadvantages: May reflect experience of one person or one clinician No explicit comparison group
  20. 20. Descriptive Studies  Populations (correlational or ecological studies)  Individuals  Case reports  Case series  Cross-sectional surveys
  21. 21. Design of cross-sectional study Defined population Exposed: Have disease Exposed; Do not have disease Not exposed; Have disease Not exposed: Do not have disease Gather data on exposure and disease
  22. 22. Cross Sectional Study Exposure Occurrence ? Time of studyDisease Occurrence ? + - + - ill exp Selection of population
  23. 23. Cross-sectional study  Also known as “prevalence” study Design Identify research question Specify target and accessible population Sample the population Measure variables of interest (usually a survey) Thus: participants classified by exposure and disease status at the same time. This allows identification of prevalent cases, calculation of prevalence rates.
  24. 24. Cross-sectional surveys  Measure variable(s) at a single time:  prevalence studies (“snapshot”)  useful for events/diseases when  chronic  common  non-fatal  temporal relationship cannot be established unless exposure permanent  If exposure unalterable, cross-sectional survey ≈ analytical study
  25. 25. Cross-sectional study  Strengths/Advantages  Can study entire populations or a representative sample  Provide estimates of prevalence of all factors measured  Standardized data collection tool.  May be quick and inexpensive  Valuable in assessing health status and health care needs of a population  Can be repeated to get trend data  Help in hypothesis generation
  26. 26. Cross-sectional study  Weaknesses/disadvantages  Information on disease and exposure collected simultaneously, therefore difficulty establishing that cause antedated effect.  Use of prevalent cases means data reflects determinants of survival as well as etiology  Cases may be misclassified due to changes in exposure or poor memory of earlier exposures  Not good for rare diseases or exposures  Cannot measure risk  Cant study temporal relationship
  27. 27. Data analysis and interpretation of descriptive studies  Cross-sectional studies and surveys are measuring prevalence  Well-suited for describing variables and their distributions – Eg. Kenya Demographic and Health Survey
  28. 28. Design of a cross-sectional study Disease No Disease Job A Job B a b c d Prevalence of disease in exposed (Job A) = a/a+b Prevalence of disease in unexposed (Job B) = c/c+d
  29. 29. Presentation of Cross Sectional Data 2x2 table Exposed Not exposed ill not ill a c b d Prevalence in exposed (Pe+) = a/ (a+b) Prevalence in non-exposed (Pe-) = c/ (c+d) Prevalence ratio = a/(a+b) / c/(c+d)
  30. 30. Job A (hazardous) 80 healthy 80 well 100 workers 20 resp 10 ill symptoms Job B (non- hazardous) 95 healthy 80 well 100 workers 5 ill 10 ill Prevalence ill job A: 20/100 = 20% Prevalence ill job B: 5/100 = 4% Prevalence ratio: 5 Cross-sectional surveys
  31. 31. Association Measures in Cross Sectional Studies Example: Corporal hygiene and trachoma Poor Hygiene Trachoma Healthy Total Yes 54 337 391 No 50 1 459 1 509 Total 104 1 796 1 900 Prevalence ratio = 13.8 / 3.3 = 4.2 Prevalence 13.8 % 3.3 % % exp 51.9% 18.8%
  32. 32. Recap Now that you have completed this session you will be able to:  Describe the differences between and descriptive and analytic studies  Describe the differences between a case report and a case series  Describe the characteristics of an ecological study  Describe a cross sectional study and explain its advantages and disadvantages  Explain the uses of the descriptive study types
  33. 33. What is the prevalence of trachoma, and is it associated with poor hygiene?  Population of 1900 Poor hygiene Trachoma Yes No Yes 54 337 No 50 1459
  34. 34. Exercise Calculate Prevalence of Chlamydia in this population of STI patients. Calculate prevalence ratio for Chlamydia among OCP users vs. non-users.

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