LESSON 9
INFORMATION BIAS
9-1 Information
Bias
·
Information bias
- systematic error due to incorrect information
·
Misclassification of exposure and/or disease can
lead to a bias in the measure of effect.
Misclassifying Disease Status
Misclassifying Exposure Status
9-2 Information
Bias (continued)
Misclassification
Probabilities – Sensitivity and Specificity
In a perfect world, no one would be misclassified: example, disease
status:
In the real world, there will be misclassification.
For misclassification of disease
Sensitivity-probability subject classified as diseased when truly
diseased
Specificity-probability subject classified as not diseased when truly
not diseased
Misclassification table for exposure status to define sensitivity
and specificity parameters.
Consider the numbers in the following misclassification table for
exposure.
1. Sensitivity for
misclassifying exposure?
2. Specificity for
misclassifying exposure?
3. Concern about
misclassification bias?
Nondifferential
Misclassification
True exposure and disease status for 2000 subjects in a case-control
study of the relationship between diet and coronary heart disease (CHD):
·
Exposure: fruits and vegetables eaten categorized as low or high
·
Disease: presence/absence of CHD; assume no misclassification of
disease
·
Assume participants overstate intake of fruits and vegetables
The following two tables describe how exposure is misclassified
separately for both the CHD cases and the non-cases.
Example illustrates non-differential misclassification of
exposure; occurs when the sensitivities and specificities do not
vary with disease status.
This example illustrates general rule about nondifferential
misclassification - bias is always towards the null.
Differential
Misclassification
True exposure and disease status for same 2000 subjects described in the
previous activity for case-control study of the relationship between diet and
coronary heart disease:
Suppose:
·
No misclassification of disease
·
Subjects over-report their intake of fruits and vegetables
·
CHD cases not as likely to over-estimate intake of fruits and
vegetables as controls
This example illustrates differential misclassification of exposure.
Sensitivities and/or specificities for misclassifying exposure vary with
disease status.
1. Bias due to misclassifying
exposure?
2. What is the direction of
bias?
In general, differential misclassification of either exposure or disease
can lead to a bias either towards or away from the null.
Independent
Misclassification of Both Exposure and Disease
Two events A and B are independent if the probability that both events
occur is equal to the product of their individual probabilities:
In some studies it may make sense that exposure and disease status
determinations are more or less independent.
Quantitative Assessment of
Misclassification Bias
Study Questions (Q9.9)
|
Question Number |
Observed Effect |
Corrected Effect |
Towards the null? |
Away from the null? |
|
a. |
2.2 |
1.7 |
|
|
|
b. |
2.5 |
3.8 |
|
|
|
c. |
4.0 |
6.1 |
|
|
|
d. |
4.1 |
1.2 |
|
|
|
e. |
0.5 |
0.9 |
|
|
|
f. |
0.8 |
0.9 |
|
|
|
g. |
0.3 |
0.2 |
|
|
|
h. |
0.7 |
0.1 |
|
|
|
Observed OR |
Sensitivity? |
Specificity? |
Corrected OR |
|
1.5 |
80% |
80% |
3.5 |
|
1.5 |
80% |
90% |
2.5 |
|
1.5 |
90% |
80% |
2.5 |
|
1.5 |
90% |
90% |
1.8 |
Correcting for
Nondifferential Misclassification of Disease
Table provides observed data from hypothetical cohort study of gender
and peptic ulcer disease:
·
Assume that gender (exposure variable) not misclassified
·
Diagnosing peptic ulcer involves some misclassification
Need reliable estimates of sensitivities and specificities for
disease. Apply extensive physical
examination on sub-sample (200 subjects) for “gold standard” determination of
peptic ulcer status:
1. Sensitivity and specificity
estimates for males?
2. Sensitivity and specificity
estimates for females?
3. Misclassification
nondifferential or differential?
Corrected cell frequencies:
4. Adjusted RR?
5. Direction of the bias?
