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

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

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

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

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

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

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

Effect of variation Within individual: 1 observer 2 observers between visits Among patients

“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

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

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

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

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

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

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

SELECTION BIAS

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

Prevalence- incidence

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 (+) History (-) Total Prevalence5816

Referral bias Sicker patients are referred to major health centers Community settingHospital setting CPNormalTotalCPNormalTotal Birth asphyxia (+) Birth asphyxia (-) Total OR

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

Healthy worker bias Exposed worker General population workersNon workers Total Death Person time Mortality rate  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.

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 (+) Respiratory disease (-) Total OR

INFORMATION BIAS

Recall bias COHORTCASE CONTROL Develop Breast Ca Did not develop Breast Ca TotalBreast Ca (+) Breast Ca (-) Total With family history of Breast Ca Without family history of Beast Ca Total Odds ratio

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).

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

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

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 [ [ [ Agree not agree

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

Definition ADefinition B Disease (+)Disease (-)TotalDisease (+)Disease (-)Total Immunization (+) Immunization (-) Total Prevalence

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.