• Feb 10th 2013, 11:09 PM
mintsharpie
Hello, I just have a quick question understanding multiple comparisons and I'd appreciate any help because I'm on the verge of failing (Worried) I'm reviewing for a test, and reading over questions and their corresponding answers in the back of the textbook. The question I'm stuck on deals with a psychologist testing the claim made by a drug company that a drug would help patients.

To do this, they selected 20 patients from their hospital, and randomly assigned them to one of four groups - group 1 receiving the new drug, group 2 receiving a different drug, group 3 receiving a different drug, and group 4 as the control group. Here is the answer given in the textbook:

http://i45.tinypic.com/np4big.jpg

I understand the first part, and how SSE is calculated and everything, but the second table with the contrasts totally baffles me. I have no idea how that table was filled in, or how I would be able to fill it in on a test if I had a different example. How were those 1s, -1s, and 2s determined? What do they mean and how were they calculated? In addition, once I go on to the appropriate post hoc test - in this case, Dunn - how do I utilise the table in terms of critical values?

• Feb 11th 2013, 06:36 PM
chiro
Hey mintsharpie.

The Contrast table is just looking at doing two sample tests for (mean_a,mean_b), (mean_a,mean_c), and (mean_a,mean_control) where (x,y) looks at x - y and tests whether there will be some kind of statistical significant effect of the two means not being equal (or not).

The contrast framework just looks at linear combinations of different treatment means and for this special case it is a difference of the other means with respect to the mean of group a.

It looks like you will need to do some kind of ANOVA/T-test/Other test on the individual means if the original H0 ANOVA was rejected (i.e. all treatment means the same) to test whether you get significant differences in pair-wise comparisons.

Also can you point out what a Dunn test is?
• Feb 11th 2013, 07:36 PM
mintsharpie
I have no idea what that means (Crying) How was the table filled in? How were those 1s, -1s, and 2s determined? What do they mean and how were they calculated? (Sadsmile)
• Feb 11th 2013, 07:39 PM
chiro
Basically a 1 represents 1*ThatMean and a -1 means -1*mean.

The row represents a 1*Mean_a - 1*OtherMean which is just a difference of two means.

A contrast is just a linear combination of means and in this special case it is just a difference of the other means with respect to mean a.

Recall that the difference of two means is something that tests whether the two means have some statistically significant difference of being equal or not equal (recall an ANOVA but in a 2 sample case it just compares the two means for inequality or equality).
• Feb 11th 2013, 08:14 PM
mintsharpie