Experimental design and the scope of inference

Learning objectives

Identify the experimental units, the explanatory variable or factor, its levels, the treatments and the response variable in a described experiment

Explain the purpose of comparing treatment groups, random assignment, replication and direct control in a well-designed experiment, and identify which of these a described experiment lacks

Explain how a control group, a placebo and single or double blinding guard against the placebo effect, and distinguish an extraneous variable from a confounding variable

Describe a completely randomized design, a randomized block design and a matched pairs design, including how the random assignment is carried out, and justify the choice of design by naming the variable used to form the blocks or pairs

Explain why random assignment of treatments supports a cause-and-effect conclusion when the difference between the groups is statistically significant, that is, too large to be explained by the random assignment alone, and state how far that conclusion can be generalized when the experimental units were volunteers rather than a random sample