"Demonstratable" — a dictionary word, or just a well known hack?

Someone has just pointed out a mis-spelling on my site - demonstratable, as in "demonstratable experience of...".

I can't see it in the New Oxford American Dictionary or the Oxford Dictionary of English. A quick Google shows it in the Merriam-Webster dictionary, but it has a note attached:

This word doesn't usually appear in our free dictionary, but the definition from our premium Unabridged Dictionary is offered here

The closest matching word seems to be demonstrable (which means the same but sounds like it shouldn't!).

My question is, is demonstratable a recognised dictionary word or just one that is generally accepted? Where do these edge-cases fit in? For example, when writing copy for a website or other literature, is it acceptable to use words like this on the assumption that, even if they are not in the dictionary, people will recognise the meaning?

Solution 1:

Don't use demonstratable.

The Corpus of Contemporary American English has 262 hits for demonstrable, and none for demonstratable. Google ngram shows a similar result:

Solution 2:

Demonstratable is not an English word, so shouldn't be used. People will probably know what you intended, just as they would if you talked about demonstratating the system; but they will never be certain whether you are joking or ignorant.

Solution 3:

Demonstratable is recorded in the OED, with examples back to 1814; though it's far less common than demonstrable.

As for whether you'd get away with it in everyday usage, the answer's yes, because -able is a productive suffix in English, at least in informal usage: in other words, you can add '-able' to lots of verbs and create adjectives with the sense 'capable of being [x]-ed'.