# i need standard deviation help

• Feb 21st 2008, 07:51 AM
benimted
i need standard deviation help
i need to do standard deviation on my calculator for my math class, but whenever i do it i get a wrong answer.

here is what i am doing
i enter the numbers in L1 and their frequency in L2 and then i do the formula

ex. 10,20,30,40,50,60 in L1
1,1,1,1,1,1 in L2

stdDev(L1L2)= 18.70828693
but the actual answer is 17.1
• Feb 21st 2008, 08:28 AM
Plato
Now I know next to nothing about calculators.
But 18.7 is the sample standard deviation, whereas 17.07 is the population standard deviation.
In some computer algebra systems the difference is Stdev() with a capital S and the other is stdev(), small s.
Look in your manual to see if the calculator does something similar.
• Feb 22nd 2008, 05:03 PM
Charbel
Bump that ^

You have population Standard deviation.

X = 10 , 20 , 30 , 40 , 50 , 50
Pr(X=x) 1/6, 1/6, 1/6, 1/6, 1/6, 1/6

Because Σ Pr(X=x) = 1 (sum of all probability = 1)

Standard deviation = σ = root(E(x^2) – [E(x)]^2)

E(x) = mean … so simply (10+20+30+40+50+60) / 6 = 35

E(x^2) = 10^2*1/6 + 20^2*1/6 + 30^2*1/6 + 40^2*1/6 + 50^2*1/6 + 60^2*1/6 = 1516.67

σ = root(E(x^2) – [E(x)]^2) = root(1516.67 – 35^2) = 17.0783 = 17.1

Population standard deviation would take a while to work out XD