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December 4th, 2015, 02:50 PM   #1
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Explain about chi-squared

Hello,
Could you explain why we have ? why alpha minus from 1 ?

The right in diagram have alpha/2 , and center is 1 - alpha, now the left of diagram 1 - alpha/2?!
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Last edited by iranch; December 4th, 2015 at 02:56 PM.
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December 13th, 2015, 06:38 AM   #2
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The total area/probability under the curve (for any probability density curve) is 1. Two values have been chosen so that the area to the left of the first line is [math]\alpha/2[/imath] and to the right of the second line is $\displaystyle \alpha/2$. Of course, the area between the two lines is $\displaystyle 1- \alpha/2- \alpha/2= 1- \alpha$.

The parts you are asking about seem to have the notation reversed. What I see in the graph is $\displaystyle \chi^2_{1-\alpha/2}(n-1)$ and $\displaystyle \chi^2_{\alpha/2}(n-1)$ when I think it should be $\displaystyle \chi^2_{n-1}= 1-\alpha/2$ and $\displaystyle \chi^2_{n-1}= \alpha/2$.

That is because, typically, the subscript is the number of "degrees of freedom" minus 1, an integer. If I am correct then the first, $\displaystyle 1- \alpha/2$ is just the area/probability to the right of the first line, 1 minus the area/probability to the left.
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