When does counterbalancing task participation order take care of practice effects? I'm thinking of a study in which participants are randomly assigned to do task A then task B, or vice-versa.
I figured there will be a problem if doing Task A is really good practice for Task B, but doing task B isn't good practice for Task A. However, let's say I have good grounds for thinking Task A's practice effect on Task B will be the same as Task B's practice effect on Task A. 
If I can assume that much am I clear to proceed? Or is there something else I should be worried about?
 A: It really depends on what you mean with "take care". 
If I reconstruct the situation you describe I assume the following:


*

*There are two tasks A and B associated with one or several measurments of response quality that form a dependent variable of interest.

*You assume that after finishing taks A, scores in task B will be elevated, but not the other way round. (This is a possible, but not a very common effect).

*You randomly assign participants to order conditions, that is AB or BA.


If what you asusme is true, then you would find an interaction between task order and task type. There is nothing wrong about proceeding this way. The design does not eliminate practice effects, but it allows you to detect them.
Still, it would be informative to know more about what you are interested in. This might be a senisble or less sensible approach depending on what your research interests are.
A: I agree with @jank's answer.  There's really nothing wrong with practice / order effects per se, and, in fact, the detection of these order effects can sometimes represent theoretically interesting contributions in their own right.
It is true that, without counterbalancing and when tasks A and B represent the within-subjects manipulation of interest, the order in which tasks A and B were presented becomes confounded with condition making inferences about the manipulation of interest ambiguous.  However, in some cases, for example when tasks A and B are two measures and the primary focus is on a between-subjects manipulation, even this is not necessarily a concern.
If you're really concerned about order effects, the safest way to approach the administration of two tasks is to do it in entirely separate sessions.  This is something my lab does routinely on tasks where we think taking one will influence future tasks.  Of course, this sort of design is not always possible.
In short, what you do with respect to order effects really depends on the specifics of the study design.  Having order effects of unequal magnitude should not be a concern in the slightest, especially if you have designed your study in such a way as to allow you to measure their magnitude.
