6 CHAPTER 1 Introduction and How to Use This Book 3. This takes us to the "Different Users in Each Group" box--we have different users in each group so we select "Y." 4. Now we're at the "3 or More Groups" box--we have only two groups of users (before and after) so we select "N." 5. We stop at the "N - 1 Two-Proportion Test and Fisher Exact Test" (Chapter 5). What Sample Size Do I Need? Often the first collision a user researcher has with statistics is in planning sample sizes. Although there are many "rules of thumb" on how many users you should test or how many customer responses you need to achieve your goals, there really are precise ways of finding the answer. The first step is to identify the type of test for which you're collecting data. In general, there are three ways of determining your sample size: 1. Estimating a parameter with a specified precision (e.g., if your goal is to estimate completion rates with a margin of error of no more than 5%, or completion times with a margin of error of no more than 15 seconds). 2. Comparing two or more groups or comparing one group to a benchmark. 3. Problem discovery, specifically the number of users you need in a usability test to find a