Today I and a colleague from the ANU supervised a group of students while they tried to identify seven species of Acacia (Fabaceae) using different types of keys. On offer were the book version of the Flora of New South Wales with its traditional dichotomous key, the online version of the same flora using pretty much the same dichotomous key, and the Lucid based Wattle Key, which is multi-entry.
I have written about dichotomous versus multi-entry keys before. The purpose of today's course was three-fold: to learn something about Acacia, as it is a very important and iconic part of the Australian flora, to become confident at using identification keys, but also to form a personal opinion on which type of key one would be more comfortable with. In other words, I encouraged the students to try both variants. I found the travails of some of them most informative, even more so than last year when we did the same course (perhaps because then I was more busy - the course was then twice as large).
A few observations:
One aspect that appeared to be particularly frustrating about the Lucid key was that if a student had gone wrong (i.e. I looked over their shoulder and informed them that the species they should arrived at had already been excluded) it was much harder to figure out how to rectify the situation. One had to look at lots of characters to see what was going on. I find it much easier to see at a glance in what part of a dichotomous key a student is, where they should go, and how to retrace their steps to where they made the mistake.
The Lucid Wattle Key in particular also seemed to be a bit overly narrow in its character table. We had Western Australian Acacia oldfieldii, and students (and I myself) would routinely measure phyllode width as 6 or 7 mm. But if anything more than 5 mm was entered into the key, the species was excluded. Similarly, I clearly observed scattered hairs on its phyllodes, but the key had them scored as glabrous. In a reasonably well structured dichotomous key one would at least end up very close to the correct species even if it made the same mistake, allowing the user to compare and figure it out. In a Lucid key, however, the true solution just disappears.
(And yes, I carefully keyed it out and checked on World Wide Wattle, and it couldn't really have been a different species: 4-merous flowers, two spikate inflorescences per node, the whole package.)
The students also had to learn to use the Lucid key in a very different way than traditional keys. They would often try to arrive at a single species, but if only two or three are left it is very hard to know which of the many available characters will still help to narrow it down further. (Yes, there is a tool for that, but it may choose extremely obscure characters, and a beginner won't know about it anyway.) A better solution is to start searching for pictures for comparison once the list has been reduced to five or less possibilities.
The obvious advantage of multi-entry keys is that they also work if the characters used in the first few questions of a dichotomous key are absent. This advantage did not come into play in our case, however, because even the dichotomous key didn't ask for fruit characters, or at least never only.