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Thanks!

- Thread starter _joey
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Thanks!

This looks like a homework question though. Our homework help policy can be found here. We mainly just want to see what you have tried so far and that you have put some effort into the problem. I would also suggest checking out this thread for some guidelines on smart posting behavior that can help you get answers that are better much more quickly.

The question asks for a proof of t=b/s(b) is equal to two-sample t-statistic with pooled standard deviation

b is least square regression with numerator and denominator parts: http://www.efunda.com/math/leastsquares/images/y1ddevsum4.gif

The question posted above is the denominator of b and I need to show that the denominator of b is equal to n0n1/(n0+n1) where n0 and n1 are sample size.

As I said the information for the part is in full. I don't require solution to the question. I seek for guidance and hints.

I'm sorry but there has to be more background than that. I don't know what the "never and ever-had steroid groups" are. Also all we're asking is that you show us what you've attempted so far.

Now [math]\sum_{i=1}^3 (X_i - \bar{X})^2 = 2[/math] but [math]\frac{n_on_1}{n} = \frac{2}{3}[/math]

So I'm thinking there has to be more to it than given because what is given doesn't work in general.

So now we know that [math]\bar{X} = \frac{n_1}{n}[/math] and we can break the sum apart into two pieces - one piece where all the X_i are 0 and one piece where they're all 1. Have you tried this yet?

So now we know that [math]\bar{X} = \frac{n_1}{n}[/math] and we can break the sum apart into two pieces - one piece where all the X_i are 0 and one piece where they're all 1. Have you tried this yet?

I guess I need to find a good paper on dummy variable in regression model to understand the concept.

Thanks for your suggestion!

I haven't tried this but it makes sense. Why is it [math]\bar{X} = \frac{n_1}{n}[/math] and not [math]\bar{X} = \frac{n_0}{n}[/math] ?

That's a good question - why don't you start by computing [math]\bar{X}[/math] for this situation (instead of just taking my word for it). Working on that simpler problem will probably help build up your intuition about working with these dummy coded variables.

What have you tried so far? You'll probably want to use the result from part (i).

I'm not sure you need that identity. I think you should apply the result of part (i) to the denominator. For the numerator just do something similar to what you did to show part (i) by breaking it up into two pieces - the parts where X_i = 0 and the parts where X_i = 1.

Play around with this for a while and see if you can simplify things.