Table 2.

Sources of bias.

Bias may be caused by

Factors related to the investigated condition

Spontaneous symptomatic improvement or fluctuation

Factors related to the trial (including statistical phenomena)

Scaling bias caused by asymmetric rating scales

Poor definition of treatment efficacy

Irrelevant or surrogate outcome measures

Conditional switching of treatment

Ascertainment or selection bias

Regression to the mean

Confounding

Factors related to assessors

Observer bias

Training bias

Factors related to the patients themselves

Response bias

Acquiescence bias

Conditioning and expectancy effects

Interactions with doctors, including suggestions and information

The ritual of treatment

Misattribution

Apprehension bias or the Hawthorne effect