BiasSyst

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Competency to be gained

from this lecturePrevent biases that may be avoided and

understand the effect of others

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Key elements

Definition of biases Selection biases Information biases Controlling the effect of biases

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Effect of misclassification

!on"differential misclassification&sually reduces the strength of associationbet'een outcome and e#posure$(( or )( closer to *%Is not a sufficient reason to dismiss the results ofa study reporting an association

Differential misclassification $bias%ay commonly increase the strength of the

associationay also reduce the strength of the association

Biases

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Types of biases

Selection biasesBiases in the 'ay sub-ects enter a study

Information biasesBiases in the 'ay information is collected afterinclusion in a study

Biases

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.evel of operation of biases

S y s t e m a t ic r e c r u i t m e n to f c a s e s / c o n t r o l s

' i t h s p e c i f ic e # p o s u r e s t a t u s

S y s t e m a t i c c o l l e c t i o n o fi n f o r m a t io n l e a n i n g t o ' a r d s

s p e c i f ic e # p o s u r e s t a t u s

C o l l e c t i o n o f i n f o r m a t i o na b o u t e # p o s u r e

I n c l u s i o no n t h e b a s i s o f o u t c o m e

C a s e c o n t r o l

S y s t e m a t ic r e c r u i t m e n to f e # p o s e d / u n e # p o s e d

' i t h s p e c i f ic r i s 0 o f o u t c o m e

S y s t e m a t i c c o l l e c t i o n o fi n f o r m a t i o n l e a n i n g t o ' a r d s

s p e c i f i c o u t c o m e s t a t u s

C o l l e c t i o n o f i n f o r m a t i oa b o u t o u t c o m e

I n c l u s i o no n t h e b a s i s o f e # p o s u r e

C o h o r t

T y p e o f s t u d y

Selection biases

Information biases

Biases

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Selection biases

Type of study Inclusion into thestudySources of selection

biases

Case control )n the basis of

outcome

Selection of casesor controls 'ithspecific e#posurestatus

Cohort )n the basis of

e#posure

Selection ofe#posed orune#posed 'ithspecific ris0

Selection biases

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E#amples of selection biases

in a cohort study Sub-ects selected on the basis of e#posure )utcome not independently distributed

among e#posed and une#posedSystematic selection of e#posed sub-ects 'ith1

+igher ris0 of outcome .o'er ris0 of outcome

Systematic selection of une#posed sub-ects 'ith1 +igher ris0 of outcome .o'er ris0 of outcome

2 b

c d

a B

c d

a b

c D

a b

C dSelection biases

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E#amples of selection biases

in a case control study Sub-ects selected on the basis of outcome E#posure not independently distributed

among cases and controlsSystematic selection of case"patients 'ith1

+igher fre3uency of e#posure .o'er fre3uency of e#posure

Systematic selection of control"sub-ects 'ith1 +igher fre3uency of e#posure .o'er fre3uency of e#posure

2 b

c d

a b

C d

a b

c D

a B

c dSelection biases

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Sources of selection biases

Surveillance Screening / diagnosis 2dmission to health care facilities Selective survival !on"response / loss to follo' up

Selection biases

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SurveillanceSystematic notification of cases e#posed

Screening / diagnosis 2dmission to health care facilities Selective survival

!on"response / loss to follo' up

Selection biases

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Surveillance Screening / diagnosis

Systematic case search among e#posed 2dmission to health care facilities Selective survival


Selection biases

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Surveillance Screening / diagnosis 2dmission to health care facilities

Systematic admission of1 Case"patients e#posed / une#posed Control"sub-ects e#posed / une#posed

Selective survival !on"response / loss to follo' up

Selection biases

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Surveillance Screening / diagnosis 2dmission to health care facilities Selective survival

Systematic inclusion of cases 'ho survived and'ho may be more or less e#posed


Selection biases

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Surveillance Screening / diagnosis 2dmission to health care facilities Selective survival !on"response / loss to follo' up

Systematic inclusion of sub-ects more li0ely toparticipate 'ho may be1 ore or less e#posed ore or less at ris0

Selection biases

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Information biases

Type of study Collection ofinformationSources of

information biases

Case control 2bout e#posure

Collection ofinformation leaningto'ards specifice#posure status

Cohort 2bout outcome

Collection ofinformation leaningto'ards specificoutcome status

Information biases

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E#amples of information biases

in a cohort study Sub-ects selected on the basis of e#posure )utcome not independently measured among

e#posed and une#posedeasurement of a higher incidence of outcome1 2mong e#posed 2mong une#posed

easurement of a lo'er incidence of outcome1 2mong e#posed 2mong une#posed

2 b

c d

a b

C d

a b

c D

a B

c dInformation biases

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E#amples of information biases

in a case control study Sub-ects selected on the basis of outcome E#posure not independently measured among

cases and controls 1easurement of a higher fre3uency of e#posure1 2mong cases 2mong controls

easurement of a lo'er fre3uency of e#posure1 2mong cases 2mong controls

2 b

c d

a B

c d

a b

c D

a b

C dInformation biases

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Sources of information biases

(ecall Investigator Data 3uality Prevarication

Information biases

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(ecallCases may recall e#posure more than controls

Investigator Data 3uality Prevarication

Information biases

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(ecall Investigator

Systematic collection of information supportinge#pected conclusions

&nconsciously Consciously

ay be e#amined in the analysis Data 3uality Prevarication

Information biases

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(ecall Investigator Data 3uality

Better e#posure data available on casesBetter outcome data available on e#posed

Prevarication

Information biases

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(ecall Investigator Data 3uality Prevarication

Systematic distortion of the truth by sub-ects

Information biases

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)ther limitations also

referred to as biases Intervention biases

!on"specific effect of interventions in a trial

2nalysis biases!on"systematic statistical analyses

Interpretation biasesInvestigators 'ith pre"conceived ideas

Publication biases!egative studies do not get published

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Chec0list to prevent biases 'hen

preparing a protocol $*/4%*5 Is the sample representative,

2re controls representative of the population

from 'hich cases are dra'n,45 2re the e#posure and outcome criteria1

Standardi6ed,Specific,

Clear,7ell measured,2pplied consistently by trained field 'or0ers,

Controlling biases

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Chec0list to prevent biases 'hen

preparing a protocol $4/4%85 Could e#posure status affect the probability

to detect the outcome,

95 Could outcome status affect the probabilityto recall the e#posure,

:5 7hat 'ill be the proportion of response,;5 7hat 'ill be the proportion of

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Discussing biases critically

(evie' critically selection and informationbiases that are li0ely to have operated

(evie' the potential effect of each of thesebiases $e5g5> under" or overestimation% (evie' 'hat can be done to address these

biases a posteriori $e5g5> analysis plan%

2naly6e possible summary effect of all thesebiases on the final results Interpret data in light of this summary effect

Controlling biases

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Ta0e"home messages

+ave a strong design that prevents biases E#amine ho' sub-ects enter a study Collect information in a standardi6ed 'ay Prevent the biases you can avoid> understand

those you cannot avoid

BiasSyst

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