order effects...

[claud08]

can anyone please define this in a clear and simple way?

[spesh-gun]

Order effects can confound experiment results when different orders are systematically (and inadvertently) associated with treatment and control conditions. A set of exam problems might be completed more quickly in one order than another, because one problem might prepare you for another but not vice versa. So if a control group of students is presented test problems in a ``good'' order and the treatment group gets the problems in a ``bad'' order, then a positive effect of treatment might be washed out by the effect of order; or, if the treatment group gets the ``good'' order, the effect of treatment (which could be zero) might appear larger than it really is.

[Glockmeister]

I wrote an example of Order Effects here:

Counterbalancing is a type of experimental design in which all possible orders of presenting the variables are included. For example, if you have two groups of participants (group 1 and group 2) and two levels of an independent variable (level 1 and level 2), you would present one possible order (group 1 gets level 1 while group 2 gets level 2) first and then present the opposite order (group 1 gets level 2 while group 2 gets level 1). This way you can measure the effects in all possible situations. Obviously there are limitations with this procedure as not all studies can be designed this way and as you increase the number of variables, conditions, etc., it just becomes logistically problematic.

Not quite.

The primary reason you would use counterbalancing is to avoid what is known as "order effects". The best way to illustrate this point is to use an example (this is pretty much the case for all the research methods)

Say I got a group of Psychology students together to complete this quiz in how happy they are. The way I intended to test this is to ask participants to complete a battery of tests. Now, if I were to give them all the tests in the same order to everyone, by the time you get to the last test you are either going to get:

- Them answering the question based on knowledge gained from doing the previous tests
- Them going "can't be fucked" and just not answering or doing the quiz properly

This introduces biases into your report, as the results you have gathered does not reflect the "true" thoughts and feelings of the participants.