Challenging Claims Re. the "Bounce" Issue

[Viett]

Under normal switch operation the digital electronics in a keyboard will filter out a small amount of natural chatter/bounce, however, in the case that an individual switch is damaged or defective, the amount of chatter/bounce can become too large to filtered.

-EliteKeyboards.com FAQ

I'm going to challenge the claim that the problem is the individual swiches. To be clear, I don't want to make Elitekeyboards/Filco look bad. It's because I had nothing better to do, so I thought I'd do some math (yes I have no life). In fact, I admire the Majestouch's (user) quick response to the problem, and that he addressed it promptly.

Here we go. Let's assume 5% of keyboards are afflicted. This number is ridiculously high, but as the math works out, this is a conservative assumption.

According to Deiz:

S, [ and enter bounce

On his FKBN104M/EB (Which Majestouch is exchanging)

Now with this information we find the probability that one individual switch is defective (calculations below). The result is 4.9308x10^-4.

Now we find the probability three or more switches are bad. The result is:

.000021032

From this, I would conclude there is something else besides faulty switches causing this problem. It is probably some filtering threshold (as Majestouch implied) that is also faulty in conjunction with the (partially?) faulty switches.

Calculations

I know the math might be a little nebulous, so feel free to question it.

[Hak Foo]

It's possible that the failures are not independent.

Say the fault is in part due to incorrect ASSEMBLY of the board.  For example, the board is stuffed in a way where the switches get damaged, or the soldering is bad that causes contact problems.  This sort of problem could affect several keys on the same board.

[Viett]

Yeah, that's very true. You have to assume independence for my calculations to apply. There's really no way to know, though.

[lowpoly]

How can you determine a probability that three switches are bad?

If there's a bad batch there's a bad batch.

Usually problems like this result from more than one cause. For ex. there might be a batch of switches with bounce times at the top of the range. The controller might debounce for low to medium bounce times.