Table 1. Four types of bias threatening internal validity.
| Type of Bias | Definition | Solution |
| Selection bias | Biased allocation to treatment groups | Randomisation; allocation concealment |
| Performance bias | Systematic differences in care between the treatment groups, apart from the intervention under study | Blinding |
| Detection (ascertainment, assessment, or observer) bias | Systematic distortion of the results of a study that occurs when the person assessing outcome has knowledge of treatment assignment. | Blinding |
| Attrition bias | Unequal occurrence and handling of deviations from protocol and loss to follow-up between treatment groups | Blinding; intention-to-treat analysis |