how to calculate frequency standard dev using GDC

How do u calculate a frequency standard deviation question.

so say it said 5 people between 50-55 years
10 people between 60-65 years

etc.

Reply 1

danniella

Hello!

If you had
Height Frequency
0 < h less than or equal to 5 5
5 < h less than or equal to 10 6
10 < h less than or equal to 15 7
15 < h less than or equal to 20 2

then you first find the midpoints (add the two numbers and half them)

Height Frequency
2.5 5
7.5 6
12.5 7
17.5 2

Then work out the mean: (2.5x5 + 7.5x6 + 12.5x7 + 17.5x2)/20 = 9

To find the standard deviation, you square each member of the "height" column. Then you find the mean of that distribution, subtract the square of the mean of the original set of data (9) and find the square root.

Height² Frequency
6.25 5
56.25 6
156.25 7
306.25 2

Mean of that is (6.25x5 + 56.25x6 + 156.25x7+306.25x2)/20 = 103.75
standard deviation = square root of (103.75 - 9²)
= square root of 22.75
s.d. = 4.77 (3sf)

love danniella

(Edited in a completely futile attempt to introduce spaces in between the end of the "height" column and the "frequency". I have typed about ten spaces but they don't show up. :s-smilie:

)

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