PURPOSE
Why did the author or organization publish the information?
If the source is trying to promote an idea or product, or persuade one thing is better than another, the source has an AGENDA with the information they present and cite. We must question that source because often times, information can be TWEAKED or SKEWED to support that side's point. The NRA and PETA are both huge groups that cite legitimate research, but they have an agenda; they're conclusions must be questioned.
If the source is non-profit, independent, and simply exists to share information, they are very trustworthy. The Center for Disease Control (CDC) isn't trying to sell anything, so they have no agenda.