Karl,

The definition and interpretation of contrasts is part of any intermediate
design of
experiments text.

Contrasts for interactions say that the effect of moving
"from level 1 of A to level 2 of A" depends on the level of B.
I will use notation YAB to indicate the levels of A and B.

For example, if

   (Y11 - Y21) differs from (Y12 - Y22)

we say that A and B have an interaction.  When A and B interact, then
the interpretation of "main effects" is ambiguous at best.  Instead,
we use the concept of simple effects, which, for example, are the
effects of changing levels of A while holding the levels of B
constant.

Interpreting the interactions themselves depends on knowing something
about the structure of the design, for example whether the effects (A
and B, here) are treatments, blocks, nested effects, or repeated
measures.

Simple effects are usually interpretable.  Interactions are tougher.

Rich
 <https://mail.google.com/a/temple.edu/?AuthEventSource=SSO#inbox>

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