ABSTRACT
Sometimes the levels of different attributes are correlated because each of the attributes is affected by a common process. For example, blurring an image decreases both sharpness and noisiness, if the noise is suprathreshold. This does not imply that the attributes are perceptually dependent, but rather is an indication that they are physically linked under some circumstances. Covariations of this type can pose difficulties for the researcher who wishes to study the impact of an attribute in isolation, because generation of samples varying only in that single attribute may be difficult or impossible.
