With a within-subject design, a major problem is that once a subject has completed the first part of the study, the experience or changed circumstances could influence performance in later parts of the study. This problem is known as a SEQUENCE or ORDER effect.

1. The Problem of Controlling Sequence Effects -- COUNTERBALANCING.
 

a. Testing once per condition (Incomplete design) -- participants are tested in each of the conditions, but only once per condition.

• Complete counterbalancing ( All possible orders)
• Partial counterbalancing (Selected orders) – (i) random selection of combinations or (ii) Latin square

b. Testing more than once per condition (Complete design)

• Reverse counterbalancing (ABBA) -- experimenter simply presents the conditions in one order  and then  presents them in the reverse order
• Block randomization -- the basic rule is that every condition occurs once before any condition is repeated a second time

2.  Problems with Biasing

a. Experimenter bias -- biased experimenters might treat the research participants in the various conditions differently.

b. Controlling for experimenter bias – standardized (mechanized) procedures; double-blind design.

c. Participant bias

•  HAWTHORNE EFFECT
• DEMAND CHARACTERISTICS
• EVALUATION APPREHENSION

3. Controlling for participant bias – deception; manipulation check; field research