Answer to Question 1
Systematic or nonsampling error may occur if the sampling units in an experimental cell are somehow different than the units in another cell, and this difference affects the dependent variable. Randomization, which is the random assignment of subject and treatments to groups, is one device for equally distributing the effects of extraneous variables to all conditions. The presence of nuisance variables will not be eliminated, but they will be controlled because they are likely to exist to the same degree in every experimental cell. Matching the respondents on the basis of pertinent background information is another technique for controlling systematic error by assigning subjects in a way that their characteristics are the same in each group.
Answer to Question 2
Experimental designs involve no less than four important design elements:
(1) Manipulation of the independent variable
(2) Selection and measurement of the dependent variable
(3) Selection and assignment of experimental subjects
(4) Control over extraneous variables
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