Question

Question on statistics:
1. What does variance and standard deviation tell us?
2. The formula for sample variance, why is it as such?? what does Xi mean?

Answer 1

\[\sum_{}^{?}(X i -Xbar)^2/n\]

Answer 2

How much time have u got?

And how much do u know about probability?

Answer 3

First,

SD^2 = Var(x) = <(x-<x>)^2>

where

<x> is the mean (by this, I mean the average, x bar = 1/N sum from i=1 to N of x_i, but where N tends to infinity, for small N, average is sufficient.)

x is a so-called "random variable" (which can be x_i, a set of numbers for small N).

Both SD and Var are measures of how "spread out" a data set is, the spread of a distribution about it's mean or average value.

Answer 4

what is their difference?

Answer 5

SD^2 = Var(x)

Answer 6

both are spread with reference to the sample mean? or population mean?

Answer 7

It will be better to speak about either a sample (small N) or a population to save specifying everything twice.

Answer 8

hmmmmm so whats the use of knowing variance and sd

Answer 9

There are many "statistics", these are just 2 of them, u can think of SD as giving more fine grained information about the sample.

Answer 10

more detailed you mean?

Answer 11

Just more....

Answer 12

hmmm alright.. can you talk to me about estimation of population parameters?

Answer 13

The simplest way to see it is to take some small samples and then play about calculating both SD and mean (Wolfram does it well, sample and population).

Answer 14

"estimation of population parameters"

U mean from samples?

Answer 15

i don't know im very confused right now with all the symbols of summation and x here and there

Answer 16

I agree all the terminology is something of a problem, nothing for it but to get grips with it....

Estimating a parameter just means taking a (hopefully) representative sample to get a statistic.

Answer 17

find the unbiased estimate of the population mean and population variance from which each of the following samples is drawn:

Answer 18

unbiased estimate of the population means what

Answer 19

Well, I would take that to mean that the sample is representative of the population.

Answer 20

ok..........

Answer 21

then they give us a lot weird things to calculate

Answer 22

I don't like calculating things...:-)

Answer 23

hmmmmm me too

Answer 24

Like I said, that's a job for software (once u understand the principles)

Answer 25

ok
say
there's this summation of (x-100) =52
summation of (x-100)^2 =93 ,
n=100
how to calculate pop mean and pop variance

Answer 26

i have to do manually..=(

Answer 27

I suggest u post that as a new question (it's not that hard).

Answer 28

new question on the board?

Answer 29

http://en.wikipedia.org/wiki/Variance
http://en.wikipedia.org/wiki/Standard_deviation

There are examples of things here u can look at...

Answer 30

i need to memorize all the formula to solve this

Answer 31

sum from i= 1 to n just means let i take the value 1,2,3,.....n

Answer 32

Any way put up a problem on the board, see if someone will help...

Answer 33

ok