Table 2.
Example of Nondifferential Misclassification Across Adjacent and Nonadjacent Categories of a Multilevel Exposure Variable
| Outcome Status (n) | |||||
|---|---|---|---|---|---|
| Exposure Status | D+ | D− | Total (n) | Risk a | RR |
| Correctly classified data | |||||
| High | 300 | 4,700 | 5,000 | 0.06 | 6.00 |
| Low | 100 | 4,900 | 5,000 | 0.02 | 2.00 |
| None | 50 | 4,950 | 5,000 | 0.01 | 1.00b |
| 30% exposure misclassification across adjacent categories (high to low)c | |||||
| High | 210 | 3,290 | 3,500 | 0.06 | 6.00 |
| Low | 190 | 6,310 | 6,500 | 0.03 | 2.92 |
| None | 50 | 4,950 | 5,000 | 0.01 | 1.00b |
| 15% exposure misclassification across adjacent categories (high to low)d | |||||
| High | 255 | 3,995 | 4,250 | 0.06 | 6.00 |
| Low | 145 | 5,605 | 5,750 | 0.03 | 2.52 |
| None | 50 | 4,950 | 5,000 | 0.01 | 1.00b |
| 30% exposure misclassification across nonadjacent categories (high to none)e | |||||
| High | 210 | 3,290 | 3,500 | 0.06 | 2.79 |
| Low | 100 | 4,900 | 5,000 | 0.02 | 0.93 |
| None | 140 | 6,360 | 6,500 | 0.02 | 1.00b |
| 5% exposure misclassification across nonadjacent categories (high to none)f | |||||
| High | 285 | 4,465 | 4,750 | 0.06 | 4.85 |
| Low | 100 | 4,900 | 5,000 | 0.02 | 1.62 |
| None | 65 | 5,185 | 5,250 | 0.01 | 1.00b |
Abbreviation: RR, risk ratio.
a Risk = number in D+/total number.
b Referent.
c 30% of people with truly high exposure are misclassified as having low exposure. In this example, the RR for high exposure versus no exposure is unchanged, but the RR for low exposure versus no exposure is biased away from the null.
d 15% of people with truly high exposure are misclassified as having low exposure. In this example, the RR for high exposure versus no exposure is unchanged, but the RR for low exposure versus no exposure is biased away from the null.
e 30% of people with truly high exposure are misclassified as having no exposure. In this example, the RR for high exposure versus no exposure is biased towards the null, and the RR for low exposure versus no exposure crosses the null.
f 5% of people with truly high exposure are misclassified as having no exposure. In this example, the RRs for both high exposure versus no exposure and low exposure versus no exposure are biased towards the null.