How to make a bad identification key

Today was my first lecture of the year, and after an introduction to the course as a whole I started with identification tools.

For the non-biologists, I have blogged before about identification keys; the most common type consists of a series of nested questions asking about characters of the organism you are trying to identify. In the style of a choose your own adventure book, answering the questions correctly will ultimately lead to your happy end, in this case the name of the species you are interested in.

Looking over my lecture brought to mind some of my own painful experiences trying to identify plants in the past. (The willow key in the Flora Europaea, argh argh argh...) I may write something more positive soon, for example explain different ways of designing a key or point towards really good examples, but for the moment let's brainstorm a list of what a taxonomist should do if they want to produce a really atrocious key.

To make them as difficult to use as possible, one should:

1. Use arcane terminology. Anybody can confound an untrained lay user of the key by writing "adaxial leaf side" instead of "upper leaf side". The real prize is to find expressions that are so specialised to your group of organisms or so local to your geographic area that only three other taxonomists on the planet will be able to understand what you mean. Prime example in Australian daisies: the "claw", which is the petiole of an involucral bract. (And non-botanists will of course not know either expression.)
2. Instead of writing questions that divide the number of remaining species neatly into two approximately equal halves, have a string of questions that divide between one single species and all others. Bonus points if the first answer to each question is a long list of characters and the alternative is always a laconic "characters not found in this combination".
3. Divide the species so that the 'distinguishing' characters are strongly overlapping, for example "glomerule diameter 1.5-2.7 mm versus glomerule diameter 2.2-3.5 mm". When faced with categorical characters, the ideal solution is something like "hairy versus naked or hairy", although a similar effect can be achieved through liberal use of "usually", "sometimes", or "except in [species that really shouldn't be in that part of the key]".
4. In a similar vein, distinguishing characters can be made less useful through the simple expedient of adding the prefix "sub-", which means kind of but not really. Examples: subglabrous = not quite entirely without hairs, subacute = a bit pointy but not really pointy, subspicate = somehow like a spike-like inflorescence but, I dunno, also a bit different? No idea really.
5. Make liberal use of characters that cannot be observed in the field, for example because you'd need a microscope. This will particularly endear you to all the park rangers and bushwalkers trying to use your key.
6. Another option is to design the key so that the first few questions are all about characters that most users will not have available. The classic example are keys to some subgroups of the daisy family (Asteraceae) or to the parsley family (Apiaceae) that focus obsessively on fruit characters. Collected a Calotis in full bloom but before fruits start to develop? Well, you are out of luck - even if in that particular genus half the species have yellow flowers and the other half have purple ones, that character is never mentioned in the key.
7. If all else fails, write a long and convoluted key that contains the same question twice, such as the key to European willows mentioned above. So you want to identify a dwarf willow from the alpine zone of the Pyrennees, and the very first question in the entire key is whether you are (a) dealing with a dwarf willow from an Arctic or alpine area or (b) a large shrub usually at least 2 m tall when mature from a lowland temperate area. You chose (a)? Silly you, of course you should have gone down the path (b) because ten or so questions in you will again be asked if you have an alpine dwarf willow, and that will then lead you to Salix pyrenaica.

The sad thing is, I am fairly sure that the colleagues in these cases I am thinking of actually thought they were writing simple and useful keys. Nobody would do these things on purpose, right?