1. Variability & Bias Yulia Sofiatin Department of Epidemiology and Biostatistics CRP I

2. Clinical scenario : An investigator wanted to study  the effect of treatment A and B  on BP reduction in patients with Hypertension The investigator asked 4 GPs to collect the data Treatment A Treatment B Normal BP High BP Normal BP High BP

3. Normal BPHigh BP Treatment Aab Treatment Bcd 15254057580 20100120 The result Cohort  Relative risk RR = (a/a+b) : (c/c+d) = 6 Case control  Odds ratio OR = ad/bc = 9

4. OR/RR < 1 : PROTECTIVE EXPOSURE OR/RR = 1 : NO RELATIONSHIP = NULL HYPOTHESIS OR/RR > 1 : HAZARDOUS EXPOSURE By definition:

5. The measurement of blood pressure depends on : -The patients Variability What kind of mistakes could occur during the study ? - The GPs - Manometer

6. Variability in Medical Research Variability within the individual Variability within population Variability related to measurement Diurnal variation Age, diet, exercise Poor calibration lack of precision Misreading, misrecording Genetic variability Environment variability

7. What are the consequences of variability? BIAS = systematic error PRECISION X random error

8. Effect of variation 80 70 60 90 100 Within individual: 1 observer 2 observers between visits Among patients

9. “To error is human” Any epidemiologic study presents many, many opportunities for error in relation to:  Selection of study participants  Classification and measurement  Comparison and interpretation

10. Epidemiologic research interpretation measurement Study participant Conclusion Decision BIAS CONFOUNDING CHANCE BIAS CONFOUNDING CHANCE

11. 1. HOSPITAL SETTING Case(CP)Control Exposed (Asphyxia) 7050120 Not exposed 305080 100100200 OR = ad/bc = 3500/1500 = 2.3

12. 2. COMMUNITY SETTING Case(CP)Control Exposed(asphyxia)7035105 Not exposed 306595 100100200 OR = ad/bc = 4550/1050 = 4.3

13. WHICH ONE IS THE TRUTH? What should we consider????

14. Bias Is a systematic error that leads to distortion of the results 1.Selection bias 2.Information bias 3.confounding

15. Most common bias Selection bias Prevalence-incidence biasSelf-selection biasReferral biasResponse biasHealthy workers biasBerkson’s bias Information bias Systematic or random errors in measurement Observer biasLoss to follow upHawthorne effectSurveillance bias Misclassification Differential Non-differential

16. SELECTION BIAS

17. COHORTCASE CONTROL Develop CAD Did not develop CAD TotalCAD (+)CAD (-)Total Highest quartile of serum cholesterol 85462547383472 Lowest three quartile of serum cholesterol 11615111627113117230 Total 20119732174151 302 Odds ratio 2.401.16 Selective survival among the prevalent cases Prevalence-Incidence bias

18. Prevalence- incidence

19. Self-selection bias Healthy (or diseased) people/ volunteers may seek out participation in the study The prevalence among volunteers in this case will be higher PILOT STUDYGENERAL POPULATION HypertensiveNormalTotalHypertensiveNormalTotal History (+) 2334 5774527753520 History (-) 3665 10147834553933 Total 5999158122362307453 Prevalence5816

20. Referral bias Sicker patients are referred to major health centers Community settingHospital setting CPNormalTotalCPNormalTotal Birth asphyxia (+) 70351057050120 Birth asphyxia (-) 306595305080 Total 100 200100 200 OR 4.32.3

21. Response Bias ResponseNon responseTotal High school students Teenage workers Total  Characteristics of those who response are different to those who do not response  association will be biased ~ self-selection

22. Healthy worker bias Exposed worker General population workersNon workers Total Death50450025007000 Person time 10009000010000100000 Mortality rate 0.05 0.250.07  The mortality rate of an exposed group of workers compared with that of the general population.  Non workers is also consist of those who are unable to work because of the disease.

23. Berkson’s bias  the combination of the exposure and the outcome under study increases the rate of admission to hospital. Community settingHospital setting Movement disease (+) Movement disease (-) TotalMovement disease (+) Movement disease (-) Total Respiratory disease (+) 1720722451520 Respiratory disease (-) 1842376256018219237 Total 2012583278423234257 OR 1.064.06

24. INFORMATION BIAS

25. Recall bias COHORTCASE CONTROL Develop Breast Ca Did not develop Breast Ca TotalBreast Ca (+) Breast Ca (-) Total With family history of Breast Ca 854625471133472 Without family history of Beast Ca 1161511162738117230 Total 20119732174151 302 Odds ratio 2.403.33

26. Recall bias  caused by differences in accuracy of recalling past events by cases and controls  There is a tendency for diseased people (or their relatives) to recall past exposures more efficiently than healthy people (selective recall).

27. Non-respondent bias; Loss to follow up  There are a possibility of different condition among:  Non-respondents to a survey from respondents  Volunteers from non-volunteers  Late respondents from early respondents,  study dropouts from those who complete the study

28. Hawthorne Bias  How will you act if you know that you are being watched?  Normal  Better  Worse

29. End-aversion bias (Measurement bias) Corruption is the most critical problem in Indonesia How will you response to: (end-of-scale or central tendency bias): respondents usually avoid ends of scales in their answers [ [ [ 1 2 3 4 5 6 Agree not agree

30. Research Question  What is the odds of suffering influenza among people with and without influenza immunization  What is the effect of different ‘case definition’  Definition A  Definition B

31. Definition ADefinition B Disease (+)Disease (-)TotalDisease (+)Disease (-)Total Immunization (+) 856114612592151 Immunization (-) 66901562669151 Total 151 302151 302 Prevalence 0.560.40.830.61

32. Misclassification bias  Caused by inaccuracy in measurement of classification of study variables.  The probability of misclassification may be the same in all study groups (nondifferential misclassification)  It may vary between groups (differential misclassification)  different ‘case definition’ between case and control groups.