3. Introduction
Bias is defined as
Any factor or process ( intentional or
unintentional) tends to deviate the results or
Conclusions of a trial systematically away
from the truth
It can be postulated that the deviation from
the true outcome of the study.
4. Where biases can happen
Study design
Data collection
Process of data analysis
Publication of study
6. Reasons of biases
Improper and deficient knowledge of the
researcher
Lack of resource, commitment and proper
encouragement
Less adherence towards research guidelines
7. Randomized control trial
Compares two or more treatment or
intervention by estimating the outcome of the
study
Participants are randomized in different groups
Best way to study the safety and efficacy of
new treatment
8.
9. Types of biases in RCT
1. Selection bias
2. Ascertainment bias
3. Hawthorne effect
4. Others
10. Selection bias
It occurs during identification of study
population
Study population not included in target
population
Randomized trial less prone to it
11.
12.
13. Ascertainment bias
One or more people involved in trial, know
about treatment
Drop out cases in case of placebo
Analyser may be biased in assessment of
outcome(observer bias)
14.
15. Hawthorne effect
Experimental effect is experiment itself and subject
at the center of attention
Manifest itself as a spurt or elevation in performance
21. Blinding
Occurs during intervention and outcome of the study
Minimizes ascertainment(performance) and
detection bias
22. Sl
no
Type of bias Methods to reduce bias
1. Selection bias Allocation concealment(blinding) hides
randomization sequence before group allocation
and continues upto outcome
Inclusion and exclusion criteria avoid
confounding results
Participants from same general population
2. Ascertainment
bias
Blinding or masking keeps people unaware about
group allocation or interventions
3. Withdrawal and
attrition bias
Intension to treat analysis includes enrolment
of all study participants
Sensitivity analysis
4. Selective
reporting bias
Double blinding
Instrument development, validation and proper
data collection
5. Publication
bias
Compulsory registration trial
23. Summary
Randomized trial, the best way to reduce
bias
Researcher should assess the risk of bias
before any trial(Cochrane approach)
Quality of evidence of the trials should be
developed by various approach (e.g- GRADE)
24.
25. References
www.cebm.net
Hernán MA, Robins JM. Causal Inference.
London: Chapman & Hall/ CRC; 2017
Heitjan DF. Ignorability and bias in clinical
trials. Stat Med. 1999;18:2421–2434
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fm1.pdf