# Thread: Sums of distributions, and variance

1. ## Sums of distributions, and variance

"For tossing a coin, rolling a die, and drawing a card from a standard 52-card deck, respective successes are defined as a head, a 6, and an ace. Find the mean and standard deviation of the total successes of:

1. 1 coin toss, 1 die roll, and 1 card draw;
2. 3 tosses, 2 rolls, and 1 draw.

The means are easy, but I can't get the right numbers for the standard deviations. And yes, I am remembering to work with variances, then square root to get back to the s.d. My respective s.d.s are 0.6469 and 1.028, but the back of the book has 0.68 and 1.05. Please help.

2. ## Re: Sums of distributions, and variance

Hey phys251.

Did you use 1/(n-1) instead of 1/n for the variance (and standard deviation) calculations?

3. ## Re: Sums of distributions, and variance

Originally Posted by chiro
Hey phys251.

Did you use 1/(n-1) instead of 1/n for the variance (and standard deviation) calculations?
Oh goodness. I'm supposed to take a SAMPLE standard deviation, not a population standard deviation, aren't I?

4. ## Re: Sums of distributions, and variance

You do if you have a sample and not a population.