CHAPTER What Sample Sizes Do We Need? Part 1: Summative Studies 6 INTRODUCTION Why Do We Care? Before tackling the question of determining the sample sizes needed by usability practitioners, we should address the question of "Why do we care?" The primary motive behind sample size estima- tion, as illustrated in Figure 6.1, is economics. If additional samples didn't cost anything--didn't take any additional time or cost any additional money--then we'd always conduct studies with very large samples. That is the case with some types of user research (e.g., Internet surveys delivered via email or conducted using Internet services such as Mechanical Turk, which has a low incremental cost typically ranging from $0.25 to $1.00 per participant). The reason that survey samples rarely contain fewer than several hundred respon-