Situation: Over 3 weeks, 46 residents from many barangays of a municipality developed acute bloody diarrhea. They had bought food from many different sources, so no list of all exposed persons exists. The public health nurse helps the municipal epidemiology and surveillance unit compare the cases with well residents (controls) on what they ate before the illness. A review of the data collection finds: Controls: chosen at random from barangay household lists; all agreed to take part Pretest: when a sample of cases and controls were questioned the same way, both groups reported their foods equally completely Cases: questioned face to face in the hospital by the nurse, who knew they were ill, and asked up to three times about each food Controls: questioned once by telephone by another staff member Which bias MOST likely inflated the odds ratio?

The two groups were questioned differently: cases were probed repeatedly, face to face, by a nurse who knew they were ill, while controls were asked once by telephone. Exposure is therefore more likely to be reported by cases, which inflates the odds ratio; this is interviewer bias, a form of differential information bias. Recall bias is ruled out by the pretest, selection bias by the random choice of controls with full participation, and nondifferential misclassification would affect both groups equally and bias the odds ratio toward 1, not inflate it.

Core concept: differential information bias in a case–control outbreak investigation

This scenario describes a classic case–control study embedded in a foodborne outbreak investigation. When the list of all exposed persons is unknown, investigators compare ill residents (cases) with well residents (controls) and ask about food exposures. The goal is to identify which food item is associated with illness by calculating an odds ratio for each exposure.

The key flaw here is that the two groups were not questioned in the same way. Cases were interviewed face to face in the hospital by a nurse who knew they were ill, and each food item was asked about up to three times. Controls were questioned once by telephone by a different staff member. This unequal intensity of questioning creates interviewer bias, which is a form of differential information bias.

Differential information bias occurs when the accuracy or completeness of exposure reporting differs between cases and controls. Because cases were probed repeatedly and in person, they were more likely to recall and report specific foods than controls who received a single telephone question. This systematically inflates the reported exposure prevalence among cases relative to controls, which in turn inflates the odds ratio away from the null value of 1.

The distinction from recall bias is important. Recall bias is a specific type of differential information bias in which cases remember exposures better because they are motivated to find a cause for their illness. However, the pretest showed that when cases and controls were questioned the same way, both groups reported foods equally completely. That finding rules out a true difference in memory or willingness to report. The problem is not the participants’ memory; it is the unequal data collection method applied by the interviewers.

Selection bias is also ruled out. Controls were chosen at random from barangay household lists and all agreed to participate. Random selection with full participation minimizes the chance that controls differ systematically from the source population that produced the cases. Therefore, the observed inflation of the odds ratio cannot be attributed to how controls were selected.

Nondifferential misclassification would occur if both cases and controls were equally likely to have their exposure status misclassified. In that situation, the odds ratio would be biased toward the null, meaning it would be pulled closer to 1 rather than inflated. Since the problem states the odds ratio was inflated, nondifferential misclassification is not the correct explanation.

Type of bias	Mechanism	Effect on odds ratio	Why ruled out or supported here
Interviewer bias	Unequal probing or questioning intensity between groups	Inflates odds ratio away from 1	Supported — cases probed up to 3 times in person; controls asked once by phone
Recall bias	Cases remember exposures better due to illness motivation	Inflates odds ratio	Ruled out — pretest showed equal reporting when questioned identically
Selection bias	Controls not representative of source population	Distorts odds ratio in either direction	Ruled out — random selection from household lists with full participation
Nondifferential misclassification	Both groups equally misclassified on exposure	Biases odds ratio toward 1	Ruled out — would not inflate the odds ratio

The broader principle from the evidence base is that information bias arises when the method of measuring exposure differs between comparison groups. In case–control studies, any systematic difference in how data are collected from cases versus controls can produce a spurious association. The educational primer on bias in clinical research emphasizes that information bias is a systematic error creating a difference between observed and true values, and that it must be distinguished from selection bias and confounding. The birth defects case–control study similarly highlights that differential participation or data collection can distort effect estimates, underscoring the need to evaluate whether observed associations reflect true relationships or artifacts of study methods.

Key point! When cases and controls are interviewed differently, the resulting bias is classified by the source of the unequal reporting. If the difference comes from the participants’ memory or motivation, it is recall bias. If it comes from the interviewer’s technique or knowledge of case status, it is interviewer bias. Here, the pretest eliminated recall bias, leaving the unequal probing as the direct cause of the inflated odds ratio.