This usually comes down to 2 questions:

• How many trees are we testing at a time?

• Are we testing the whole tree, or just part of it?

How many trees?

If we’re testing an existing tree for problems, before starting our IA redesign, the answer here is simple – we’re testing just the one tree.

If we’re revising the IA for a site, and we haven’t done a baseline test yet, it’s a good idea to test the “before” and “after” versions. At minimum, this means testing two trees – the existing one (to get a baseline score) and our revised tree (to see what improved and what didn’t).

As mentioned above, though, we really should be testing more than one alternative, so we can be sure our eventual new tree is as effective as possible. Typically, we’ll test 2-3 proposed trees against each other (and against the existing baseline tree), then we’ll test a “best of” hybrid of the two in a second round.

For more on testing alternative trees against each other, see The design phase: creating new trees in Chapter 3.

Which part of the tree?

If we’re testing a small or medium-sized tree (say, less than 500 items), we will normally test the whole tree – no major pruning required.

If our tree is larger (say, 500-1000) items, we have 2 options:

• Test the whole tree – Easy to prepare, but affects how many tasks we can ask each participant.

• Test a “pruned” version of the tree – Takes some effort on our part, but lets us concentrate on the parts we’re really interested in.

Finally, if our tree is very large (more than 1000 items), testing the whole tree may be feasible, but in most cases we recommend pruning the tree to keep the participants’ effort from becoming onerous.

For more on pruning trees, see Which part of the tree? in Chapter 6.

Next: Who will we test?