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Definition of bias in research studies: 

"Systematic error [is] introduced into sampling or testing by selecting or encouraging one outcome or answer over others."

Types of bias can also include:

• Selection bias - Research that relies on recruiting or selecting participants may result in selection or participant bias in a number of ways. For instance, participant recruitment might unintentionally target or exclude a specific population, or researchers may not appropriately account for participant withdrawal.
• Design bias - Researchers may engage in poorly designed research, which could increase the likelihood of bias. Poor research design may occur when the research questions and aims are not aligned with the research methods, or when researchers choose a biased research question.
• Funding bias occurs when the results of a scientific study positively support the study's financial sponsors. In this case, the research may be skewed so that the funding organization can have a study that supports whatever outcome they want. 
• Publication bias - Not all research articles are published. Publication or reporting bias occurs when publishers are more likely to publish articles showing positive results or statistically significant findings. Research showing negative results may be equally important to the contribution of knowledge in the field but may be less likely to be published.
• Location bias -The publication of research findings in journals with different ease of access or levels of indexing in standard databases, depending on the nature and direction of results. 
• Citation bias - The citation or non-citation of research findings, depending on the nature and direction of the results.
• Language bias - The publication of research findings in a particular language, depending on the nature and direction of the results
• Outcome reporting bias  - The selective reporting of some outcomes but not others, depending on the nature and direction of the results.

Sources: Table 10.1.a: Definitions of some types of reporting biases and Chapter 7: Considering bias and conflicts of interest among the included studies | Cochrane Training