Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I have a financial calculator and am trying to use it to calculate the standard deviation when probability is involved.

For a standard deviation question say on the numbers $1$, $5$, $6$, $10$

Press 1, E+
Press 5, E+
Press 6, E+
Press 10, E+
Press Stat Button - ox

Answer $3.2$

However I have just been presented with a question in the format

$$\begin{array}{l|l} \text{Probability}&\text{Score}\\ \hline 0.1&1\\ 0.3&5\\ 0.4&6\\ 0.2&10 \end{array}$$

With the forumla:

$$\text{stdev}^2 = \sum (\text{Probability} \times (\text{Score} - \text{Average Score}))^2$$

How would I do this on my calculator?

share|cite|improve this question
up vote 1 down vote accepted

You have three choices:

1)make the list 1,5,5,5,6,6,6,6,10,10 which has the correct probabilities and do the same as you are doing

2) Calculate average score with one pass through the data, avg score=0.1*1+0.3*5+0.4*6+0.2*10=6, then make a second pass through the data doing the equation you present stdev^2=0.1*(1-6)^2+0.3*(5-6)^2+0.4*(6-6)^2+0.2*(10-6)^2 (note your square is one layer too far out-Probability should not be squared)

3)One one pass accumulate sum(data) and sum(data^2), then use stdev^2=(sum(data))^2-sum(data^2)

share|cite|improve this answer
It's a shame to not have this feature, for ease of memory I've just learnt your first option, which is not ideal but works! – Pez Cuckow Sep 3 '11 at 10:38

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.