A flaw in measuring exposure or outcome data that result in different quality (accuracy) of information between comparison groups. (Aschengrau & Seage, p. 270) Different types of observation bias include:

1. Recall bias: a result of differential level of accuracy in the information provided by compared groups. (Aschengrau & Seage, p. 271)
2. Interviewer bias: systematic differences in soliciting, recording, or interpreting information that occurs in studies using in-person or telephone interviews.
3. Differential and non-differential misclassification