Bias is the presentation of evidence in a manner which supports
the author's opinion. It does not necessarily make information
good or bad, but it is something of which to be aware. In
general, one expects an honest person to examine the evidence
and then form an objective opinion rather than forming an
opinion first and then selecting evidence to prove his or her
opinion. However, many people adhere to political,
philosophical, or theological positions which color their
perception of evidence. This is why it is important to consider
the authority of the author.  Some biases are simple to
identify. A pharmaceutical company which publishes research on
its medications is obviously going to be favorable to its own
products because it wants to sell them. In a situation like this
you may want to look for research conducted by an outside party.
A pro-choice group is going to present a different picture of
abortion than will a pro-life group. If you want to examine both
sides of the issue, you will want to find info