Demonstratable - a dictionary word, or just a well known hack?

Question

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?

Answer 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:

Answer 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.