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The answer here is the same as for most other forms of user testing: ideally, we prefer fresh participants because they haven’t seen the tree (in an earlier revision) or the tasks before, so they should be untainted (so to speak).
If we have a large audience to draw from, we can certainly do some screening to make sure we’re not accepting people who participated in an earlier round of tree testing. (For more on screening, see
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Screening for specific participants in Chapter 9.)
In the real world, however, we often do not have a huge pool for testing, so we often forego this screening just to get the minimum numbers we need to produce clear results. We think this is acceptable because the “experienced” participants don’t really have a big advantage over the fresh participants – the tree may have changed substantially since the last test, the tasks may have been revised, and enough time has probably passed (say, a week or two) that their recall is only partial.
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