Answer: Objectivity/Bias is the major criterion that he missed.
Explanation: Many websites published by organizations will promote that organization's agenda or political viewpoint on the topic. The NRA is not in favor of gun control, and it promotes gun ownership as a 2nd Amendment right.
In order to balance his information and present alternate points of view, he could find sources with the opposite bias, or find statistics from studies done by unbiased sources, such as statistics reported on state or national government sites.
Authority/Credibility is another criterion to check. Can the source be trusted to provide the sources of its information: studies done by other credible, unbiased authors, Can the information be verified by finding similar information on other sites or in reputable publications?