When making decisions about which technologies to use, you need evidence about which options are effective to make the best possible use of your technology budget. Many information sources, from marketing material to peer reviewed studies published in prestigious journals, present evidence of product effectiveness. The quality of this evidence can vary widely. This guide describes four key types of evidence you are likely to encounter and explains how to tell whether these types of evidence can provide strong support for claims about effectiveness. The types of evidence in this guide are ordered from weakest to strongest, and each evidence description is accompanied by examples of information sources containing that type of evidence.