The first question would be does the word start with A-M
Second would narrow search down to max 7 letters
third would narrow search down to max 4 letters
fouth would narrow search down to 2 letters and
fifth question (at most) gives us first letter of the word.
Would remaing questions always guess the word?
Are there any words that share the first 15 letters with 2 or more other words?

The first question would be what is the word

I'm not thinking of a specific word STnapse just seing if the forum members can disprove the premise

Antidissestablishment
And
antidissestablishmentantarianism

Osual rules of 20 questions apply. Yes/No answers only.

Sorry I misread wait a sec

I thought about something like that, but at what point in those words shouldn't you be asking a yes/no question? Once you have Antidisses next letter must be a 't'.

1. Is the word "Bob Genghiskhan"?

Damn it redhouse, you have a window into my soul.

Well, if you assume the best case where you can find a perfect question every time that would reduce the the number of potential words by a factor of 2, you could identify up to 2^20 = 1,048,576 words. If we are to believe this study cited on Wikipedia - "In December 2010 a joint Harvard/Google study found the language to contain 1,022,000 words and to expand at the rate of 8,500 words per year. The findings came from a computer analysis of 5,195,769 digitised books. Others have estimated a rate of growth of 25,000 words each year.", it really seems unlikely that the hypothesis holds at this moment and we can say for sure that it will be false in a few years time.

Whether you believe Harvad and Google or not is larglely immaterial. The Oxford English Dictionary itself contains just 171,476 active words (and 47,156 obsolete words & 9,500ish dirivative words), and doesn't expand by anything like 8,500 per year.

Of course, using my trusty Collins Pocket Dictionary (and I so no good reason why we shouldn't) one can find the correct word far easier.

@Octavious: how so?

That's to your last sentence.

2^21 would do the trick, and what if we don't use all the scientific balderdashian neoisms? 2^18? lol

To prove the premise wrong you need a big bunch of really long words that have to be asked almost every lettre because there are more options...

The size of the words doesn't really matter, you could just make a "dictionary" with one word on each page and always ask "is the word in the first half?" and then discard whatever half the word is not in. Do this 20 times and you can identify 1,048,576 pages (words), no matter how long or how much alike they are.

Agreed with Alex. You could simplify the problem by just numbering every word and do a binary search, cutting in half each time. The length of the word wouldn't matter, because it would just be reduced to its position in the dictionary. I play this game with my kids to teach them about binary searches and how quickly you narrow things down by cutting the possibilities in half. but in my example I tell them I can guess any number up to 1000 in 10 guesses (2^10 = 1024, so I have a little buffer)

Thanks guys

Ok I'm thinking of a number between 1 and 1000 phil, go.

You are thinking of pi, what did I win?