Edit: thanks to spoon (see post below this one), I found several errors.
Quote Originally Posted by clvacva
Not sure if this is a correct way of checking this but I used Flopzilla selected 98s to see how much it hits.
Interesting. I dl'd flopzilla, and found what appears to be an error in its calculations for 98s. It lists the probability of quads on the flop as 0.03%. There are exactly 2 ways to flop quads: 999 and 888. The total number of flops is 19,600. My calculator says the probability (in decimal form) is:

.0001020108

or, in percentage form:

0.0102%

Edit: mistake was leaving out 4 straight flushes (>=quads) as spoon said.

Hmm...not sure why, but flopzilla's overestimating the probability of quads by 3x-ish. So...I checked their calculation on Full Houses, which can occur in 18 ways. Flopzilla says the probability is 0.122% when the actual probability is 0.09184%.

The only way a Full House can flop is if all three flop cards are 8's or 9's. Since there are 3 8's and 3 9's left in the deck, the total ways to draw 3 of them on the flop is "6 choose 3" or 20 ways, but two of them are quads (see above). Then the probability is 18/19600.

Edit: found that error now - there's a "cummulative" option and "absolute" option. When you check the correct one, you get the correct output.

I just deleted the rest of my post. I TOLD y'all I suck at discrete mathematics.