We have also already noted that attributes are typed by the sort of values they may accept. We list here some typical examples.
Values may be binary. For example, asking whether a spelling checker allows the user to create his own personal dictionary leads to an attribute whose value is simply `yes' or `no'.
Values may be nominative or classificatory. For example, we might have an attribute which asked how a new word could be added to a spelling checker dictionary. The range of possible values would be inside checker, outside checker, not possible.
Values may be comparative. For example, there is evidence that `guessability' is an important factor in user acceptance of a new piece of software: if he can guess how a particular function works or what a particular icon stands for on the basis of his past experience without having to look it up, he will be happier with the software. It is hard to imagine a measure for guessability other than a simple comparative measure which awards, say, a score on a rating scale.
A value may also be numerical. For example, we might ask not only whether a user can define his own personal dictionary, but also how many personal dictionaries the spelling checker will accept.
A value may also be a metric. For example, we might ask in what percentage of cases where an error is detected does the spelling checker give the correct suggestion as the first suggestion offered.