# Homework Help: Standard deviation

1. Nov 17, 2013

### EdmureTully

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

I have to calculate the standard deviation

2. Relevant equations

http://0.tqn.com/d/statistics/1/0/M/-/-/-/standarddev.GIF

3. The attempt at a solution

0.64 0.4096
2.06 4.2436
0.16 0.0256
3.24 10.4976
1.74 3.0276
1.06 1.1236
4.04 16.3216
1.56 2.4336
1.3 1.69
3.64 13.2496
3.16 9.9856
3.04 9.2416
0.86 0.7396
3.56 12.6736
2.94 8.6436
1.74 3.0276
2.96 8.7616
2.34 5.4756
6.46 41.7316
2.76 7.6176

total = 151.9204

uncorrected sd = 2.75
corrected sd = 2.83

1 89.20
2 86.50 2.70
3 88.40 1.90
4 91.80 3.40
5 90.30 1.50
6 87.50 2.80
7 92.60 5.10
8 87.00 5.60
9 89.80 2.80
10 92.20 2.40
11 85.40 6.80
12 91.60 6.20
13 87.70 3.90
14 85.00 2.70
15 91.50 6.50
16 90.30 1.20
17 85.60 4.70
18 90.90 5.30
19 82.10 8.80
20 85.80 3.70

the answer they get is 2.91

average is 88.56

did i make any calculation error?

I can't believe i made any calculation error

2. Nov 18, 2013

### vela

Staff Emeritus
Yes, you made some errors. Your value for the mean is correct. One of your differences is off, and you compounded that error by not summing the squares accurately.

It would have helped if you described what you were doing instead of just posting arrays of unlabeled numbers and leaving it to others to decipher what you did.

3. Nov 18, 2013

### EdmureTully

Ok, thanks. One last thing, do I have to divide by n-1 or just n?

4. Nov 19, 2013

### vela

Staff Emeritus
For an unbiased estimator, you need to divide by n-1.

5. Nov 19, 2013

### Ray Vickson

What on earth is your data? Is the first data set a list of 40 numbers (presented as two lists of 20, or is it two separate lists, each of size 20? If it is a single list of size 40 I get a total of 210.1804; if it is two lists of size 20 each I get a total of 49.26 for the first list and a total of 160.92094 for the second list. None of these match what you claim---but your analysis is totally meaningless because you do not give any clues as to what you are doing.