Control in Experimental Designs

Control u Key element of experimental and quasi- experimental designs –Subjects (one group or several) on which no variable (e.g., treatment) is applied »Hold a variable or condition constant –Establishes baseline to compare changes in treatment groups –Attempts to reduce the effect of confounding independent variables

Example - Confounding Variable u Does the “contract-relax” technique lead to an increase in hamstring length? –Simultaneous treatments »Contract-relax »Local heat to hamstrings »Acupuncture u Local heat and acupressure would be confounding variables –There is a need to control for these variables

Strategies for Gaining Control u Matching - narrow selection criteria (e.g., age, gender, similarities in a particular condition) will limit potential for error when comparing control vs. experimental groups u Sampling method - a key factor in establishing control –Random assignment eliminates bias - makes groups as comparable as possible

Strategies for Gaining Control u Double blind –Observer has no knowledge of subject group –Subject has no knowledge of placebo vs. treatment u Single blind –Subject has no knowledge of treatment vs. placebo u Blinding –Observer and/or subject’s knowledge of treatment may bias outcomes –Blinding hides: »Observer’s knowledge of subject assignment (control vs. experimental) »Subject’s knowledge of treatment (placebo vs. experimental)

Strategies for Gaining Control u Placebo tends to make subjects feel they are receiving a treatment or intervention being studied thereby establishing control by eliminating potential bias –Caution: Deception by using a placebo may be challenged by the IRB

Strategies for Gaining Control u Subjects as their own control –Subjects are matched to themselves –Exposed to all levels of independent variables »Control treatment or condition »Experimental treatment of condition u Inherently a repeated measures design

Strategies for Gaining Control u Analysis of covariance (ANCOVA) –A traditional way to statistically gain control –Partitions extraneous confounding variables »Treated as a covariate –Controls for initial differences between groups »Effect is to adjust scores on the dependent variable for pretest differences between groups u Statistically establishes equivalence

Example - Covariate u Mean pretest measures between groups are likely to be different –Older subject’s soft tissues often become less elastic with age u Treat subject age as a covariate u Measure maximal hamstring length using a “sit-and-reach” test in a community- sponsored fitness program before and after a 10 weeks exercise program u Groups –Children ages –Adults ages