Can someone here help me set up a probability calculation?

[Bucky]

What I'm trying to figure out is, what are the chances of something that has a 25% chance of occurring happening 41 times out of 48 attempts. How would I set up that problem?

[Jackpine Radical]

1. Try this:

http://stattrek.com/Tables/Binomial.aspx

The answer it gives me is 2.03E-18, or

p=.00000000000000000203

Does that help?

[Bucky]

3. Thanks! I tried that stat trek site, but couldn't remember my college stats formulas

It was 2 kids getting the same wrong answers over and over on a final. I wanted to be able to say something technicalish and impressive at the inevitable parent conference. A one in 50,000,000,000,000,000 chance of this occurring by accident is making me doubt their honesty.

[HopeHoops]

2. That's a bullet hitting a smaller bullet probability.

There's a major difference between asking "what's the chance of rolling a six now" and then asking it again vs. "what's the probability of rolling a six twice in a row."

In the first case, the answer is 1 in 6 and the chance on the second roll is 1 in 6. Asking "twice in a row" is 6^2.

If I understand your question correctly, you would need a 1:4 probability 41 times out of 48 attempts. I believe that would be 4^48 - 4^7 - a shitload. That comes out to 1 in 79228162514264337593543933352. Just slightly better than winning the lottery. :evilgrin:

It has been 25 years since I took my last probability & statistics class, so I might be a little rusty, but that's at least in the ballpark.

[Bucky]

4. One in 79 octillion? I dunno. Could be a coincidence.

Jackpine's numbers came out to one in 50 quadrillion, give or take. All I know is that my Excel program can't handle it.

[HopeHoops]

5. Excel has less power than the calculator program that came with Win95.

I'm not kidding. It is more limited in calculation capacity.

[dnbn]

6. Prob = n!/((n-k)!*k!)*p^k*(1-p)^(n-k)

= 48!/(41!*7!)*0.25^41*0.75^7 = 2.0324*10^-18 (n=48, k=47, p=0.25).

[ladjf]

7. So, what was the probability? nt

[struggle4progress]

8. Just do the indicated arithmetic. I get

2.03244E-18