# Sums of distributions, and variance

• Oct 31st 2013, 03:08 PM
phys251
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.
• Oct 31st 2013, 07:26 PM
chiro
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?
• Nov 3rd 2013, 12:39 PM
phys251
Re: Sums of distributions, and variance
Quote:

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?
• Nov 3rd 2013, 02:07 PM
chiro
Re: Sums of distributions, and variance
You do if you have a sample and not a population.