Designing Experiments Section 5.2 Part 1
Explanatory Variable Any variable that explains the response variable. Often called an independent variable, predictor variable, or factor.
Response variables The outcome of a study. A variable you would be interested in predicting or forecasting. Often called a dependent variable or predicted variable. Advertising: What are the results of repeated exposure to an advertising message? The treatment in the study of repeat exposure to advertising. Combining the levels of the two factors forms six treatments.
Basic Principles of Statistical Design Control the effects of lurking variables on the response, most simply by comparing two or more treatments. Replicate each treatment on many units to reduce chance variation in the results. Randomize – use impersonal chance to assign experimental units to treatments.
Control A simple experiment in a controlled laboratory environment often has this form. Treatment---› Observed Response In field experiments, this design can be too simple Placebo effect: many patients respond favorably, to any treatment, even a placebo. A placebo is a dummy treatment. Control group: The group of subjects that receive the dummy or placebo treatment.
Caution!!! Don’t confuse control and control group. Control refers to the overall effort to minimize variability in the way experimental units are obtained and treated.
Replication Even with control, there will still be natural variability or systematic differences between the groups. If we assign many units to each group, the effects of chance will average out. Use enough subjects to reduce chance variation.
Randomization The use of chance to divide experimental units into groups. Cell phones and driving: Does talking on a hands-free cell phone device distract drivers?
Completely Randomized Design When all experimental units are allocated at random among all treatments, the experiment is said to have a completely randomized design.
Helper-Hinderer In a study reported in a November 2007 issue of Nature, researchers investigated whether infants take into account an individual’s actions towards others in evaluating that individual as appealing or aversive, perhaps laying the foundation for social interaction (Hamlin, Wynn, and Bloom, 2007). In one component of the study, sixteen 10-month-old infants were shown a “climber” character (a piece of wood with “google” eyes glued onto it) that could not make it up a hill in two tries. Then they were shown two scenarios for the climber’s next try, one where the climber was pushed to the top of the hill by another character (“helper”) and one where the climber was pushed back down the hill by another character (“hinderer”). The infant was alternately shown these two scenarios several times. Then the child was presented with both pieces of wood (the helper and the hinderer) and asked to pick one to play with. The color and shape and order (left/right) of the toys were varied and balanced out among the 16 infants. http://www.youtube.com/watch?v=anCaGBsBOxM