Good control conditions, like fine food and wine, are easy to admire and
hard to produce. It helps to be aware of some common
pitfalls:
four effects
are particularly relevant to experimental AI. They are called
ceiling, floor, regression, and order effects, respectively, and
spurious effects, collectively. Spurious
effects suggest that a treatment is effective when it isn't, or suggest
that a treatment isn't effective when it is. Ceiling and floor effects are
sometimes easy to see, regression and order effects can be quite subtle.