# mean and standard deviation

• Apr 1st 2009, 10:34 PM
dvsdars@gmail.com
mean and standard deviation
A new young executive is perplexed at the number of interruptions that occur due to employee relations. She has decided to track the number of interruptions that occur during each hour of her day. Over the last month, she has determined that between 0 and 3 interruptions occur during any given hour of her day. The data is shown below. (10 points)

Number of Interruptions in 1 hour Probability
0 interruption .40
1 interruptions .30
2 interruptions .20
3 interruptions .10

(a) On average, how many interruptions per hour should she expect?
(b) What is the standard deviation of numbers of interruptions per hour?
• Apr 1st 2009, 10:40 PM
mr fantastic
Quote:

Originally Posted by dvsdars@gmail.com
A new young executive is perplexed at the number of interruptions that occur due to employee relations. She has decided to track the number of interruptions that occur during each hour of her day. Over the last month, she has determined that between 0 and 3 interruptions occur during any given hour of her day. The data is shown below. (10 points)

Number of Interruptions in 1 hour Probability
0 interruption .40
1 interruptions .30
2 interruptions .20
3 interruptions .10

(a) On average, how many interruptions per hour should she expect?
(b) What is the standard deviation of numbers of interruptions per hour?

What formula have you been taught for calculating expected value?

What formula have you been taught for calculating variance? Standard deviation?
• Apr 1st 2009, 11:17 PM
dvsdars@gmail.com
I don't know the formula. I took stats a couple years ago and a friend asked me to help him out. That's the only question I couldn't answer. He didn't give me any formulas for solving it.
• Apr 2nd 2009, 01:23 AM
mr fantastic
Quote:

Originally Posted by dvsdars@gmail.com
I don't know the formula. I took stats a couple years ago and a friend asked me to help him out. That's the only question I couldn't answer. He didn't give me any formulas for solving it.

Then what has your friend been taught? I doubt your friend would be asked to do these calculations if he had not been taught the formulae.

Read this: Discrete Random Variables: Mean or Expected Value and Standard Deviation

Standard deviation is square root of variance. And variance is $E(X^2) - [E(X)]^2$. And $E(X^2)$ is calculated in the same way as E(X).