When researchers investigate programmer behavior, a messy area involving human factors, their methods must become ever more sophisticated. Above all, we must be aware of how much we can learn and where we must accept limits to our knowledge. Therefore, this section discusses the principles that help us find software engineering data we can trust and apply it with appropriate caution to new situations. The chapters touch on some applications of the data, but focus on broader lessons that you can take to other contentious issues in software engineering.