# Homework Help: Product Moment Correlation coefficient

1. May 9, 2013

### songoku

1. The problem statement, all variables and given/known data

2. Relevant equations
$$r=\frac{S_{xy}}{S_{x} S_{y}}$$

$$S_{x}=\sqrt{\Sigma(x-\bar{x})^2}$$

3. The attempt at a solution

I have calculated the value of Sx several times and I got 43.3 but the answer key said that Sx = 11.2

1. Is my formula correct?
2. If yes, is my calculation correct?

Thanks

2. May 9, 2013

### Staff: Mentor

43.3 is larger than the largest deviation from the average, it cannot be the standard deviation.
I think there is a prefactor missing in your formula for S_x.

3. May 9, 2013

### songoku

I just found out that my formula is wrong.

What do you mean by "43.3 is larger than the largest deviation from the average" ? Please explain because I really want have that kind of insight

Thanks

4. May 9, 2013

### Staff: Mentor

The standard deviation is a measure for the "typical", or "average"* deviation from the mean. You have a mean somewhere at ~30 (guessed), so the largest deviation from the mean is about 24. The typical deviation cannot be larger than this.

*squared average, to be precise

5. May 11, 2013

### songoku

I get it. Thanks a lot for your help