What percentage of registered participants eventually show up at conferences? I am organizing a small workshop which is over subscribed. We will have to reject a few applications. The worst applicants are easy to spot, but there are a few borderline cases, and I'm afraid that a few unfair decisions will necessarily have to be made.

To minimize the damage, I would like to know how many of the registered participants who we accept will actually show up, so that if I am aiming at X actual participants, could I safely accept X + dX?

I have noticed that several name tags always are left unclaimed at small meetings, because of people not showing up. I would like to know how to estimate how many people I can expect not to show up.

I am looking for answers from people who have organized (preferably small) meetings and have first-hand knowledge about the typical numbers.

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Edit:

So the week of the workshop came and I can give you attendance numbers. It is good that I didn't overbook since we got 100% attendance (49/49 registered participants). This is despite the event being free of charge (some people suggested the attendance would be lower for free events). Some of the things that we did that may have helped secure such good numbers are thorough screening of applications and asking for confirmation upon acceptance of the applicant. For instance, we weeded out a few applicants who were unlikely to show up unless we paid for their travelling and a couple of accepted applicants told us that they would not be able to attend, a couple of months in advance of the workshop.
I am afraid that, especially for a small meeting, the variation of how many do not show up will be too large to be useful. For example, if I am organizing a 50-person event I would expect that anything from 2 to 10 would be a "normal" number of no-shows. That is, if you really can't have more than, say, 50 participants you can't really safely overbook at all. However, note that oftentimes a few participants more don't really hurt, but of course this depends on your infrastructure and plans for the meeting.

Some further considerations:


Generally, free events have a very large number of no-shows (sometimes in the 50%+ range, from what I have heard) while paid ones do not. This has led to some meetings I know introducing a nominal attendance fee, just to make sure that people think a little before signing up. Some other meetings have also tried to levy a fee only if a registered participant does not show up, but I am not sure how well this works and if it is worth the administrative and inter-personal hassle.
Note that no-shows are often not independent events, statistically speaking. For example, if you have a bunch of participants from the same group / team and they have an unexpected important deadline come up, they all will cancel at the same time.
If you have a very small meeting and you know (some) of your participants well you can attempt to use this to make a stab at estimating no-shows. For instance, I once planned an event for about 30 people, and I knew that some of the senior professors who initially expressed interest where &lt;50% to actually come. I also knew that for some fraction of the industry participants an important client meeting may come up making them cancel last-minute. Based on this information I decided that 35 registrations for a 30-person meeting was fairly safe (we ended up being 28).