1. Different Types of Epidemiologic Studies
Kamran Yazdani, MD MPH
Department of Epidemiology and Biostatistics
School of Public Health

2. CASP Critical Appraisal Skills Programme http://www. phru. nhs
CASP Critical Appraisal Skills Programme (Appraisal Tools)

3. CASP Major Points: Is the Objective CLEAR?
Is the Method (Design & Analysis) APPROPRIATE?
Dealing with:
Selection Bias
Information Bias
Confounding
Chance error
Reporting (Analysis, Results)
Interpreting (Association, Causation)
Generalizability
Consistency, Coherence
SCREENING

4. http://www.strobe-statement.org/Checklist.html (Checklists)
STROBE
STrengthening the Reporting of OBservational studies in Epidemiology
(Checklists)

5. Epidemiology
The study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to control of health problems.

6. Aim of epidemiological studies
To determine distribution of disease
Descriptive Studies
To examine determinants of a disease
To judge whether a given exposure causes or prevents disease
Analytical Studies

7. انواع مطالعات
توصيفي: با هدف بررسي و توصيف يك وضعيت بدون آنكه قصد بررسي يك رابطه (آزمون فرضيه) را داشته باشيم
تحليلي: با هدف بررسي يك رابطه ، اختلاف يا ارتباط (آزمون فرضيه) صورت مي پذيرد

8. Epidemiologic Design Strategies
Descriptive studies
case reports, case-series, cross-sectional
Analytical studies
Observational studies
Cross-sectional
Case-control studies
Cohort studies
Intervention studies
Clinical trials
Field trials
Community trials
Experimental (animals)
Diagnostic Tests studies or Process Research

9. انواع مطالعات توصيفي تحليلي مشاهده اي گزارش مورد گزارش موارد كوهورت
مداخله اي
گزارش مورد
گزارش موارد
كوهورت
كارآزمايي باليني
اكولوژيك
مورد شاهدي
كارآزمايي ميداني
مقطعي
كارآزمايي اجتماعي

10. Advantages of Case Reports and Case series
Allows for the description of new disease processes.
Allows for the description of outcomes associated with rare diseases or rare features of any disease.
To formulate hypotheses of the association

11. Disadvantages/Limitations of Case Report & Case Series
Impossible to determine disease frequency.
Cannot establish causality between exposures or risk factors and disease outcome.

12. Case Report example
In 1961, a published case report of a 40 year-old woman who developed pulmonary embolism after beginning use oral contraceptive

13. Case Series example
In Los Angeles, five young homosexuals men, previously healthy, were diagnosed with pneumocyst cariini pneumonia in a 6-month period (80-81)

14. Case Series example
RESULTS: Twelve patients with histopathologically confirmed tumours detected after extraction of teeth were studied. There were 11 males and one female giving a male to female ratio of 11:1. They ranged in age from years with a mean age of 53 years. Pain and swelling were the most common presenting complaints. The mandible was more often involved seven (58.3%) cases while five (41.7%) cases occurred in the maxilla. Squamous cell carcinoma (in 9 cases) was the most common malignant neoplasm among these patients.

15. Cross-sectional studies
Cross-Sectional Studies measure existing disease and current exposure levels.
They provide some indication of the relationship between the disease and exposure or non-exposure

16. Cross Sectional Studies (contd)
Sample without knowledge of Exposure or Disease
Sample at one point in time
Mostly prevalence studies/surveys

17. Cross Sectional Studies
(Advantages)
Good design for hypothesis generation
Can estimate overall and specific disease prevalence and sometimes rates
Can estimate exposure proportions in the population
Can study multiple exposures or multiple outcomes or diseases

18. Cross Sectional Studies
(Advantages)
Relatively easy, quick and inexpensive!!!
Best suited to studying permanent factors (breed, sex, blood-type) to deal with TEMPORALITY.
Often good first step for new study issue

19. Cross Sectional Studies
(Disadvantages)
Impractical for rare diseases
Not a useful type of study for establishing causal relationships
Confounding is difficult to control
Problems with temporal sequence of data
hard to decide when disease was actually acquired
miss diseases still in latent period
recall of previous exposure may be faulty

20. Cross Sectional Studies
Disease Status
Yes No
Total
Exposure
Status
Yes a b
a +b No c d
c +d
a +c
b +d N

21. Cross Sectional Studies
Depression
Yes No
Total
Yes 3 87 90
low SES No 14 75 89 17
162
179

22. Cross Sectional Studies
Points need concern:
Sampling
Protocol (Quality Assurance & Control)
Appropriate Analysis
Appropriate and Fair Interpretation

23. Randomized Clinical Trials
Population
Outcome
Inclu & Exclu
New Treatment
Improved
Not improved
Participants
(Randomization)
Comparison treatment
Improved
Not improved

24. Randomized Clinical Trials
Cure
Total
Yes No A 13 87
100
Treatment 75
100 B 25 38
162
200

25. Cohort / Follow-up Studies
Study population
(Non-diseased)
Exposed
Non-exposed
Disease +
Disease -

26. Cohort study, at a glance
Case control
Cohort
Study group
Diseased/ healthy
Exposed/ unexposed
Measure of effect
OR, AR
Risk, RR,OR.AR
temporal
Hard to establish
Easy to establish
multiple
exposures
outcomes
time
Short
Long
cost
inexpensive
Expensive
Population size
small
Large
Information bias
exposure
Outcome
Best when
D rare E frequent
E rare D frequent
Problems
Control selection
Exposure information
Unexposed selection
change over time

27. جدول توافقي در مطالعه‌ي كوهورت
Disease Status
Yes No
Total n1
Exposure
Status
Yes a b
a +b No c d
c +d n2
a +c
b +d N

28. آيا مطالعه كوهورت هميشه تحليلي است؟

29. مطالعه كوهورت ـ انواع زمان حال مواجهه پي آمد مواجهه پي آمد مواجهه

30. Prospective vs. retrospective Cohort Studies
Prospective Cohort Studies
Time consuming, expensive
More valid information on exposure
Measurements on potential confounders
Retrospective Cohort Studies
Quick, cheap
Appropriate to examine outcome with long latency periods
Admission to exposure data
Difficult to obtain information of exposure
Risk of confounding

31. چه موقع مطالعه كوهورت مناسب‌تر است؟
شواهدي موجود باشد
LOSS TO FOLLOW UP را بتوانيم كنترل كنيم
مدت پي گيري نسبتاً كوتاه باشد
بتوانيم كوهورت تاريخي انجام دهيم
مواجهه نادر باشد

32. Selection of the Exposed Population
Sample of the general population:
Geographically area, special age groups, birth cohorts (Framingham Study)
A group that is easy to identify:
Nurses health study
Special population (often occupational epidemiology):
Rare and special exposure

33. Selection of the Comparison Population
Internal Control Group
Exposed and non-exposed in the same Study population (Framingham study, Nurses health study)
Minimise the differences between exposed and non-exposed
External Control Group
Chosen in another group, another cohort (Occupational epidemiology: Asbestosis vs. cotton workers)
The General Population

34. Bias Selection bias: Information bias
Non-representativeness (unequal in e+ & e-)
Non-response during data collection
Losses to follow up
Healthy worker effect
Information bias
Misclassification on exposure
Misclassification on event

35. CASP questions

36. Case-control study Exposed Cases Non-exposed Study Population Exposed

37. Direction
exposure
outcome

38. Applications Diseases with long latency period
For best use of time and money
The best for rare diseases & useful for prevalent diseases
Mutiple exposures

39. Case-control study, at a glance
Cohort
Study group
Diseased/ healthy
Exposed/ unexposed
Measure of effect
OR, AR
Risk, RR,OR.AR
temporal
Hard to establish
Easy to establish
multiple
exposures
outcomes
time
Short
Long
cost
inexpensive
Expensive
Population size
small
Large
Information bias
exposure
Outcome
Best when
D rare E frequent
E rare D frequent
Problems
Control selection
Exposure information
Unexposed selection
change over time

40. BIASes
Selection
Information

41. Selection of cases
Case definition is more important than other studies:
Strict diagnostic criteria (high degree of caseness)
Homogenous diseases (one well-defined outcome or health-related state)

42. Selection of cases Sources of cases
Population
Hospital (available, better dx, low inf bias, high sel bias)
Registry …
Are the cases representative of total population or a fraction of it?

43. Selection of cases Incident vs. Prevalent
Selection & Information BIASes

44. Selection of controls Study base Deconfounding Comparable accuracy
Sel bias
Deconfounding
confounding
Comparable accuracy
Info bias

45. Selection of controls سوال:
آيا مي توان براي يك گروه بيمار كه از بيمارستان انتخاب شده اند، گروه كنترل را از جامعه گرفت؟

46. Types of controls Population controls Hospital controls
Friend, RDD, neighbor controls
Hospital controls
Similar disease as controls

47. Hospital controls Similar study base Similar quality of information
Convenience

48. Population controls Tax lists, vote lists, telephone directories,…
RDD:
Selection bias due to higher ses., families with more than phone lines and family size

49. Matching

50. Ratio of controls to cases
Statistical considerations (increasing power)
Unsuitable for very low or very high powers under equal sizes

51. Ratio of controls to cases
Control to case ratio: up to 4-fold:
case cont 1: 1: 1: 1: 1:

52. جدول توافقي در مطالعه‌ي
مورد ـ شاهدي
Disease Status
Yes No
Total
Exposure
Status
Yes a b
a +b No c d
c +d
a +c n1
b +d n2 N

53. CASP questions

54. مقايسه RR و OR
در مطالعه كوهورت OR مي تواند برآورد خوبي از RR باشد اگر:
بيماري بروز بالايي نداشته باشد
در مطالعه مورد شاهدي OR مي تواند برآورد خوبي از RR باشد اگر:
موردها نماينده بيماران جامعه باشند
شاهدها نماينده سالمهاي جامعه باشند

55. Comparing cohort and case control
Study group
Diseased/ healthy
Exposed/ unexposed
Measure of effect
OR, AR
Risk, RR,OR.AR
temporal
Hard to establish
Easy to establish
multiple
exposures
outcomes
time
Short
Long
cost
inexpensive
Expensive
Population size
small
Large
Information bias
exposure
Outcome
Best when
D rare E frequent
E rare D frequent
Problems
Control selection
Exposure information
Unexposed selection
change over time

56. Intuition and Logic in Research
Dominant Mental Activity
Intuition
Feeling
Judgement
Experience
Analysis
Experiment
Clinical trials
Cohort Study Hi
Case-control Study
Cross-sectional Study
Control over variance
Case Report
Case Series
Qualitative
Research Lo
Potential for Misinterpretation Lo Hi