You want to use [-1 0 1 zeros(1,6)]
This will test if the conditions increase over the three conditions in a linear manner. 

If the values don't sum to zero, which they don't have to, then you will be testing whether the weighted sum of the 3 conditions is greater than 0. This is a very different question.

In terms of the oddness of leaving out the middle term. The value of condition 2 will have no impact on the linear relationship. To try this yourself. Do the following in excel.
Set A1=1, B1=5
Set A2=2, B2=rand()
Set A3=3, B3=10

Now, make an X Y scatterplot, add a trendline with equation. Now keep resetting B2 and you will notice that the slope does not change. Thus, condition 2 will not have an impact on the slope. And the slope can be thought of as the condition3-condition1 (which would be [-1 0 1 0 0 0 0 0 0])


Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Postdoctoral Research Fellow, GRECC, Bedford VA
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Office: (773) 406-2464
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On Fri, Jul 8, 2011 at 11:25 AM, Joshua Balsters <[log in to unmask]> wrote:
Apologies if this is already answered on the list, I found this question posted a couple times but without a response.

I have a GLM with 9 conditions. Three of these conditions increase parametrically so I'd be using a contrast vector [1 2 3 zeros(1,6)]; however i believe that t-contrasts are supposed to sum to zero so I'm not sure this contrast vector is correct. If you sum these to zero then you get [-1 0 1 zeros(1,6)]; which also doesn't seem right.

How important is it that t-contrasts sum to zero? and if it is important what's the appropriate contrast for an odd number of linearly increasing conditions?

Thanks in advance
Josh