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# Help standard variance watch

1. Hello, I'm not too sure how to use to formula

in order to work out the standard variance of a sample.

Say my sample goes:

1-10 cm 8 people
11-20 cm 12 people
21-30 cm 7 people

and the mean is 15.3.

Thank you
2. bump?
3. Would it be just

(8 - 15.3)^2 + (12-15.3)^2 + (7-15.3)^3
________________________________ __

(8+12+7)

= 32.13?
4. Your data is given as a frequency distribution.
Do you have a version of this formula for frequency distributions? Involves f...
5. (Original post by vc94)
Your data is given as a frequency distribution.
Do you have a version of this formula for frequency distributions? Involves f...
I think this is it:
6. This formula is equivalent:

sample variance = (n/(n-1))*( (sum of f*(y^2))/n - (mean)^2 )

Where y is the midpoint of each group and f is the group's frequency.
7. (Original post by vc94)
This formula is equivalent:

sample variance = (n/(n-1))*( (sum of f*(y^2))/n - (mean)^2 )

Where y is the midpoint of each group and f is the group's frequency.
Could you please use the numbers in the example to show how it'd work? I'd really appreciate it. Thanks.
8. Bump?
9. Isn't this the unbiased estimate of the variance from a sample?
10. (Original post by hollywoodbudgie)
Hello, I'm not too sure how to use to formula

in order to work out the standard variance of a sample.

Say my sample goes:

1-10 cm 8 people
11-20 cm 12 people
21-30 cm 7 people

and the mean is 15.3.

Thank you
if this was a table, the measurements would be column x, you would have to work out the mid-class values for column x before you can work with them. I would make a few more columns, the frequency can be f, you would have an xf column and an x^2 x f column, and the totals at the bottom, then I would use Vc94's formula because its easier to use
just plug in values, should be easy.

11. The midpoints of the groups are 5.5, 15.5 and 25.5

So calculate:

I make your mean 15.129 ?

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