study designs


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study designs

  1. 1. Study Designs 1
  2. 2. Objectives Understand the concept of epidemiological studies. Understand the concepts of exposure, outcome and risk. Identify the association between exposure and outcome. Quantify the magnitude of association by calculating and interpreting the measures of association:  Relative risk or risk ratio  Odds ratio 2
  3. 3. Important study questions1. What? 1. Event Surveillance data sources2. When? 2. Time Surveillance,3. Where 3. Place ? Descriptive studies 4. Person4. Who? 5. Reason Why? Analytical studies5. 6. Way6. How? 3
  4. 4. Concepts  Variable  Outcome or Effect variable  Measures of disease  Risk Factor or Exposure variable  Measures of risk  Association of risk and disease  Measures of association 4
  5. 5. Types of Study designs Non Intervention Studies  Descriptive studies  Comparative (analytical) studies  Exploratory studies Intervention Studies The two categories of intervention studies are:  Experimental studies  Quasi-experimental studies. 5
  6. 6. Current Classification E P ID E M IO L O G IC A L S T U D Y D E S IG N S D E S C R IP T IV E A N A L Y T IC A L 1 . C a s e R e p o rt O B S E R V A T IO N A L E X P E R IM E N T A L 2 . C a s e S e rie s CASE CONTROL 1 . S in g le B lin d 3 . C o rre la tio n / E c o lo g ic a l COHORT 2 . D o u b le B lin d4 . C ro s s S e c tio n a l / P re v a le n c e 3 . T rip le B lin d 6
  7. 7. E p id e m io lo g ic a l S t u d ie s I n te r v e n tio n a l s tu d ie s N o n -in te r v e n tio n a l S tu d ie s o r O b s e r v a tio n a lC lin ic a l F ie ld C o m m u n ity E x p lo r a to r y D e s c r ip tiv e T r ia ls I n v e s tig a tio n I n te r v e n tio n C a s e s e r ie s C r o s s -s e c tio n a l s u r v e y s A n a ly tic a l A n a ly tic a l S tu d ie s C r o s s -s e c tio n a l C a s e -C o n tr o l C ohort c o m p a ra tive
  8. 8. Association between exposureand outcomeExposure Outcome Cause Effect 8
  9. 9. Descriptive Studies Descriptive studies involve the systematic collection and presentation of data to give a clear picture of a particular situation and can be carried out on a small or large scale. Case studies Case series 9
  10. 10. Comparative or Analytical Studies An ANALYTICAL STUDY attempts to establish causes or risk factors for certain problems. This is done by comparing two or more groups, some of which have or develop the problem and some of which have not. 10
  11. 11. Cross-sectional Comparative studies Many cross-sectional surveys focus on comparing as well as describing groups. For example, a survey on malnutrition may wish to establish:  The percentage of malnourished children in a certain population  Socio-economic, physical, political variables that influence the availability of food 11
  12. 12. Cross Sectional SurveysQuantify the distribution of certain variables in a study population at a point of time. They may cover, for example: Physical characteristics of people, materials, or the environment,etc 12
  13. 13. Cross Sectional Surveys The behavior of people and the knowledge, attitudes, beliefs, and opinions that may help to explain that behaviour (KAP studies), or events that occurred in the population. Cross-sectional surveys cover a sample of the population. If a cross-sectional study covers the total population it is called a census. 13
  14. 14. Case Control Studies In a CASE-CONTROL STUDY, the investigator compares one group among whom a problem is (e.g., malnutrition) with another group, called a control or comparison group, where the problem is absent to find out what factors have contributed to the problem. 14
  15. 15. 15
  16. 16. Odds Ratio (OR) Measure of the strength of the association between risk factor and outcome. The derivation of the Odds Ratio is based on three assumptions: - The disease being investigated must be relatively rare. - The cases must be representative of those with the disease - The controls must be representative of those without disease 16
  17. 17.  A study was conducted to find out the association of smoking to lung cancer. 100 cases of lung cancer were interviewed about their smoking status and 60 of them were smokers. 200 Normal people were also interviewed and 40 of them were smokers. Find the odd ratio in the given scenario and interpret your result 17 as well.
  18. 18. Biopsy ResultsSmoking CA Lung CA Lung Total status Positive Negative a+b Yes a b c +d No c d a +b +c + d Total a+c b+d 18
  19. 19. Biopsy ResultsSmoking CA Lung CA Lung Total status Positive Negative a+b Yes 60 40 c +d No 40 160 a +b +c + d Total 100 200 19
  20. 20.  Odd ratio = a/c ÷ b/d = a/c x d/b = 60 x 160 40 x 40 = 6 Interpretation: Lung cancer patients are six times more likely to be smokers than normal persons 20
  21. 21. Cohort Studies In a COHORT STUDY, a group of individuals that is exposed to a risk factor (study group) is compared with a group of individuals not exposed to the risk factor (control group). The researcher follows both groups over time and compares the occurrence of the problem that he or she expects to be related to the risk factor in the two groups to determine whether a greater proportion of those with the risk factor are indeed affected. 21
  22. 22. What is a cohort? Cohort - Latin word for one of the 10 divisions of a Roman legion A group of individuals  sharing same experience  followed up for a specified period of time Examples  birth cohort  occupational cohort chemical plant workers 22
  23. 23. 23
  24. 24. Cohort Study DiseaseExposure Study starts occurrencetime 24
  25. 25. Cohort Study DiseaseStudy starts Exposure occurrencetime 25
  26. 26. Cohort Study Notexposed Incidence among Non exposedexposed Incidence among exposed 26
  27. 27. Steps in designing CohortStudies Identify group of exposed subjects Identify group of non-exposed subjects Follow-up both groups for disease Measure frequency of disease occurrence in both groups Compare risks between exposed and non-exposed group 27
  28. 28. Relative Ratio/Risk (RR) Ratio of incidence of the disease (or death)among exposed and the incidence among non-exposed. It is a direct measure (or index) of the “strength” of the association between suspected cause and effect 28
  29. 29.  A scientist wanted to study the effect of smoking on lung cancer. He enrolled 500 people who were smoking and the 1000 individuals who were not smokers. After 15 years of follow up he found that among the smokers 50 developed lung cancer while among the non smokers 25 developed lung cancer. Find the relative risk of lung cancer among smokers and interpret the result as well 29
  30. 30. Biopsy Results CA Lung CA LungSmoking status Total Positive Negative a+b Yes a b c +d No c d a +b +c + d Total a+c b+d 30
  31. 31. Biopsy Results CA Lung CA LungSmoking status Total Positive Negative 500 Yes 50 b 1000 No 25 d a +b +c + d Total a+c b+d 31
  32. 32.  Relative risk = Incidence of disease Among Exposed Incidence of disease among non exp RR = a/a+b ÷ c/c+d = 50/500 ÷ 25/1000 = 50/500 x 1000/25 = 4 Interpretation: Smokers are 4 times more likely to develop lung cancer than non smokers 32
  33. 33. Exploratory Studies An Exploratory study is a small- scale study of relatively short duration, which is carried out when little else is known about a situation or a problem. 33
  34. 34. Intervention Studies In intervention studies, the researcher manipulates a situation and measures the effects of the manipulation. Usually (but not always) two groups are compared, one in which the intervention takes place (e.g.. treatment with a certain drug) and another group that remains "untouched" (e.g., treatment with a placebo) . 34
  35. 35. Experimental Studies Experimental design is the only type of study design that can actually prove causation. In an EXPERIMENTAL STUDY, individuals are randomly allocated to at least two groups. One group is subjected to an intervention or experiment, while the other group(s) is not. The outcome of the intervention (effect of the intervention on the dependent variable/problem) is obtained by comparing the two groups. 35
  36. 36. Experimental studies ExposedExposure Diseaseassigned occurrence Not exposed Unethical to perform experiments on people if exposure is harmful 36
  37. 37. Past Present Future Case-control studyExposure  Outcome Prospective Cohort study Exposure  Outcome Retrospective Cohort studyExposure  Outcome Cross-sectional study Exposure Outcome
  38. 38. Classical Experimental Study Design The classical experimental study design has three characteristics: Manipulation Control Randomization 38
  39. 39. Quasi – Experimental Studies In a QUASI-EXPERIMENTAL STUDY, at least one characteristic of a true experiment is missing. One of the most common quasi-experimental designs uses two (or more) groups, one of, which serves a control group in which no intervention takes place. Both groups are observed before as well as after the intervention, to test if the intervention has made any difference. 39
  40. 40. Tutorial An epidemiologist wants to find out the association of bottle feeding with the incidence of diarrhoea. He enrolled 100 children fed through bottle and 80 experienced at least one episode of diarrhoea during the follow up period. Out of 150 on breast feeding 60 had experienced diarrhoe during the same period. Calculate the relevant measure of risk Interpret your result 40
  41. 41. Thank you 41