It’s often the case that getting access to participants is extremely difficult. For example, we might be interested in whether a new interface to a PET scanner reduces the time it takes attending radiologists to adjust a setting on the scanner.If we had access to one of these users, we could ask them to perform a task on the existing software version three times, record how long it took to complete, then have them attempt the same task three times on the new software, and finally have them attempt it again three times on the old version. A graph of what this might look like is shown below.