Date: Nov 30, 2012 11:09 PM
Author: Ray Koopman
Subject: Re: En passant question: What if a plot of slope CI<br> ’s is lousy, but splits the “m’s” perfectly?
On Nov 30, 7:54 am, djh <halitsk...@att.net> wrote:

> Consider the following plot of slope CI?s (paste into fixed font

> document if it wraps):

>

> ----------m----------

> a1,1,C

> --------------------m--------------------

> b1,1,C

> ---------m---------

> c2,1,C

> -----m-----

> b47,1,C

> -----m----

> c1,1,C

> -----m-----

> a3,1,C

> -----m----

> c2,1,S

> -----m------

> c1,1,S

> ------m-----

> a1,1,S

> -----m-----

> b47,1,S

> ----------m----------

> a3,1,S

> ------------------------m------------------------

> b1,1,S

>

> Between ?C? and ?S? slopes, there is obviously no CI split at all.

>

> But there is a perfect split of ?m?s?.

>

> Can?t anything be concluded from the perfect split of ?m?s??

>

> (I think I know what your answer will be but wanted to check

> nonetheless.)

If all the true slopes were identical and the sampling distributions

of the sample slopes were symmetric then the probability of having

all C < all S, or vice versa, would be 2*6!^2/12! = 1/462 = .0022,

so it certainly looks like there's a subset effect. It might be more

to the point to do S vs C t-tests for each fold separately. And, in

the same spirit, for each cell in the Fold x LengthInterval design.

(Those are the same heteroscedastic t's you've been doing.

Don't give the data to Excel and tell it to do a t-test.)