This document discusses bias and confounding in epidemiological studies. It defines bias as systematic errors that lead to incorrect estimations of exposure-outcome associations. Selection bias and information bias are two main types of bias. Confounding occurs when a third variable is associated with both the exposure and outcome. The document then describes biases that can occur in cohort and case-control studies specifically, such as selection bias, information bias, and confounding bias. It concludes with ways to eliminate bias, including using a representative sample, randomization, and blinding, as well as methods to control for confounding like restriction, matching, stratification, and multivariate analysis.

1. Presented by :Ikram Ullah
BS MLT,2nd batch
KMU, Peshawar.
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2. Contents
 Bias and its types
 Confounding
 Bias in cohort study
 Bias in case control study
 Elimination of bias
 Control of confounding
 References
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3. Quiz
The analytical study where population is the unit of study?
a) Cross sectional
b) Ecological
c) Case-control
d) Cohort
 A longitudinal or prospective study is also referred to as
a) Ecological study
b) Cross sectional study
c) Cohort study
d) Observational study5/17/2017 3

4. Bias
 “Any systemic error (design, data collection, analysis
or reporting of a study) in epidemiological study that
results in incorrect the estimation of the association
between exposure and outcome”
 “Deviation of results or inferences from the truth”
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5. Types of bias
 1. Selection bias
• Occurs when the two groups being compared
differ systematically
• That is, there are differences in the characteristics
between those who are selected for a study and
those who are not selected
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7. Types of selection bias
i. Volunteer bias
ii. Berskson’s bias
iii. Exclusion bias
iv. Inclusion bias
v. loss/withdrawal to follow up
vi. Non response bias
vii. Healthy worker effect
viii.Selective survival
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8. 2.Information bias
 Method of gathering information is inappropriate and
yields systemic errors in the measurement of exposure
or outcome
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9. Types of information bias
i. Observer bias
ii. Recall bias
iii. Horthorne effect
iv. Surveillance bias
v. Lead time bias
vi. Misclassification bias
a) Differential
b) Non differential
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10. Confounding
 A situation in which the measure of effect of
exposure on disease is distorted because of the
association of the study factor with other factors
that influence the outcome
 Confounder must be…….
1. Risk factor for the disease independently
2. Associated with exposure under study
3. It should not be in the direct chain or link
between the exposure and outcome5/17/2017 10

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12. Bias in cohort study
1. Selection bias : Select participants into exposed
and not exposed groups based on some
characteristics that may affect the outcome.
 Loss to follow up bias: Some subjects in any case
are likely to be lost to follow up/drop out
e.g. smoking and lung cancer, loss to follow up
bias occurs if smokers who have lung cancer are
more likely to be lost(die) than non smoker with
lung cancer
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13. Cont….
• Cross over bias: This may happen because those
having the exposure (e.g. smokers) may cross over
to the non exposed group(i.e. become non
smokers)and vice versa.
• Healthy worker effect: Healthy people remain
workers, whereas those who remain unemployed,
retired or disabled are a group of less healthy
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14. Cont….
2. Information bias:
• Observer bias : This occurs because the
investigator is aware about the fact as to which
subject is ‘exposed’ and who is not exposed. For
obviating this, if possible, ‘blind’ the observer to
the exposure status, the details of exposure being
known only to another co - worker
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15. Cont….
 Measurement bias: Collect different quality and
extent information from exposed and not exposed
groups
 For eliminating this , inform all the subjects of
both groups well in advance of the dates and
timings of medical examination
 Ensure that both the groups are examined by
observer who have similar type of training and
using similar type of technique and instrument5/17/2017 15

16. Bias in case control study
1. Selection bias:
 Selective survival: Only surviving subjects
availabe to be studied; those surviving differ from
those dying potentially important ways.
 Solution: rapid case ascertainment and interview
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17. Cont…
 Selection of inappropriate cases or control:
Cases or control who do not have adequate
chances of exposure .
 For example in a study of OC use (exposure) and
thrombophlebitis (cases),cases or control who
have undergone hysterectomy or using some other
contraceptive , will not have adequate chances of
exposure
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18. Cont….
 Information Bias:
 Occurs due to flawed data collection
procedures.
 Types of Information bias –
 Recall bias
 Interviewer bias
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19. Cont…..
Recall bias : Cases who are aware of their
disease status may be more likely to recall
exposures than controls
e.g. mothers who have given birth to babies with
congenital malformation, mothers with normal
babies
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20. Cont….
• Interviewer bias: When interviewer is not
blinded knows case status of subjects there is
potential for interviewer bias.
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21. Cont…..
 Confounding bias:
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22. Eliminating of bias
 Representative sample
 Well defined structure research
 Blinding
 Randomization
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23. Control of confounding
 At design stage:
1. Restriction: Subject chosen for study possessing
a narrow range of characteristics . e.g. restrict
study to women having a least one child.
2. Matching: For each patient in one group there is
one or more patients in comparison group with
same characteristics, except for the factor of
interest. E.g. age ,sex ,race etc.
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24. Cont…
3. Randomization: Subjects of study are randomly
selected to even out unknown confounders.
At analysis stage:
1. Stratification: The process of separating a sample
into several sub-samples according to specified
criteria such as age ,sex etc.
2. Multivariate analysis: The statistical analysis of
data collected on more than one variable. E.g.
people age ,weight, body fats .5/17/2017 24

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