# Tying to Find Variance and St Dev

Printable View

• Feb 10th 2011, 05:16 PM
Peyton Sawyer
Tying to Find Variance and St Dev
my variance is -2.37583E+12

i need to find St Deviation but i cannot square root this number

can someone help me change this number so i can put it under a square root?

thanks
• Feb 10th 2011, 05:35 PM
harish21
Firstly, you should know that variance is always non-negative. Here are some PROPERTIES if you want to have a look. So your proclaimed variance of
-2.37583E+12 does not make any sense.

Could you please post your actual question?
• Feb 10th 2011, 05:38 PM
ymar
But if we change the number, we will get the wrong result, won't we?

You can't square root this number because it's a negative number. Negative numbers don't have square roots in real numbers. Are you sure your variance is correct?

Sorry Harish, I started writing when there were no replies yet. I don't know how to delete this post.
• Feb 10th 2011, 05:44 PM
harish21
Quote:

Originally Posted by ymar
But if we change the number, we will get the wrong result, won't we?

You can't square root this number because it's a negative number. Negative numbers don't have square roots in real numbers. Are you sure your variance is correct?

Sorry Harish, I started writing when there were no replies yet. I don't know how to delete this post.

We cannot delete our own posts. Only Moderators can. It often happens that multiple number of people are replying to a post at the same time. Besides, your post is informative. So there is no need to delete it!!
• Feb 10th 2011, 05:52 PM
Peyton Sawyer
Code:

x (cash prize $) P(x) X*P(X) X2*P(X) 15000000 6.8E-09 0.102 1530000 200000 0.00000028 0.056 11200 10000 0.000001711 0.01711 171.1 100 0.000153996 0.0153996 1.53996 7 0.004778961 0.033452727 0.234169089 4 0.007881463 0.031525852 0.126103408 3 0.01450116 0.04350348 0.13051044 0 0.972682422 0 0 0.298991659 1541373.131 Mean Variance St Dev 0.2989917 -2.37583E+12 ? • Feb 10th 2011, 05:53 PM Peyton Sawyer oh my goodness it squeshed all me data togeher :( sorry • Feb 10th 2011, 07:40 PM CaptainBlack Quote: Originally Posted by Peyton Sawyer Code: x (cash prize$)                    P(x)                  X*P(X)                    X2*P(X) 15000000                            6.8E-09                  0.102                            1530000 200000                                    0.00000028                  0.056                            11200 10000                                    0.000001711                  0.01711                    171.1 100                                    0.000153996                  0.0153996                    1.53996 7                                    0.004778961                  0.033452727                0.234169089 4                                    0.007881463                  0.031525852                0.126103408 3                                    0.01450116                  0.04350348                    0.13051044 0                                    0.972682422                  0                            0                                                           0.298991659                    1541373.131                                                     Mean              Variance                St Dev                                0.2989917              -2.37583E+12        ?

You are using the wrong formula for the variance.

$Var(X)=E(x^2)-\overline{x}^2$

CB