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Calculator giving wrong answers when calculating standard deviation :s

I have a TI-84 Plus, and when calculating standard deviation using it it gives the wrong answers.

For example, it says the standard deviation of 1,2,3,4,5,6 and 7 is 2.160246899 when the actual answer is 2.

All other values are similarly abit too high.

Does anyone have any ideas why this is, and how to fix it?

Reply 1

Mathmoid

You are calculating the sample standard deviation, the calculator appears to be calculating an estimator of the population deviation from the sample. Are you using the correct calculator function?

\sigma_n^2 = \frac{\sum{x^2} - n\bar{x}^2}{n}

\sigma_{n-1}^2 = \frac{\sum{x^2} - n\bar{x}^2}{n-1}

Reply 2

guitar_chick

The calculator only has the one function for standard deviation :frown:

Bit of a pain when it cost nearly £100...

Reply 3

nota bene

Isn't there a difference in using 1-var-stat and 2-var-stat?

Reply 4

Kolya

guitar_chick

The calculator only has the one function for standard deviation :frown:

Bit of a pain when it cost nearly £100...

I am sure that it can calculate the s.d. of a population as well as a sample. Have you consulted the manual? :smile:

Reply 5

Mathmoid

Assuming that your cacl is something like the TI84 plus, then according to here:
http://education.ti.com/guidebooks/graphing/84p/TI84PlusGuidebook_Part2_EN.pdf

the statistical variable known as Sx is the sample s.d. and the variable known as