Bias: "a systematic error, or deviation from the truth, in results or inference" (Cochrane Handbook, ch. 8)
Types of bias include:
- Publication, time lag, or multiple publication bias
- Location bias
- Citation bias
- Language bias
- Outcome reporting bias.
For more details on bias and how to prevent it, see: Cochrane Handbook for Systematic Reviews of Interventions (2008,Chapter 8 & Chapter 10, table 10.1.a)
For guidance on assessing study types, see the assessment templates from the NIH's National Heart, Lung, & Blood Institute forms.