Experimental Design and Blocking
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The document discusses blocking variables in an experiment that investigates whether energy gels make runners faster. The experiment will block for professional status by: 1) Dividing the sample into pro and amateur athletes 2) Randomly assigning pro athletes to treatment and control groups 3) Randomly assigning amateur athletes to treatment and control groups 4) Ensuring pro/amateur status is equally represented across groups Blocking controls for characteristics that experimental units already have to reduce extraneous variation between groups.
Experimental Design and Blocking
- 1. + Experiments Slides edited by Valerio Di Fonzo for www.globalpolis.org Based on the work of Mine Çetinkaya-Rundel of OpenIntro The slides may be copied, edited, and/or shared via the CC BY-SA license Some images may be included under fair use guidelines (educational purposes)
- 2. More on Blocking We would like to design an experiment to investigate if energy gels makes you run faster: ● Treatment: energy gel ● Control: no energy gel It is suspected that energy gels might affect pro and amateur athletes differently, therefore we block for pro status: ● Divide the sample to pro and amateur ● Randomly assign pro athletes to treatment and control groups ● Randomly assign amateur athletes to treatment and control groups ● Pro/amateur status is equally represented in the resulting treatment and control groups Why is this important? Can you think of other variables to block for?
- 3. Difference Between Blocking and Explanatory Variables Factors are conditions we can impose on the experimental units. Blocking variables are characteristics that the experimental units come with, that we would like to control for. Blocking is like stratifying, except used in experimental settings when randomly assigning, as opposed to when sampling.
- 4. More Experimental Design Terminology... Placebo: fake treatment, often used as the control group for medical studies Placebo effect: experimental units showing improvement simply because they believe they are receiving a special treatment Blinding: when experimental units do not know whether they are in the control or treatment group Double-blind: when both the experimental units and the researchers who interact with the patients do not know who is in the control and who is in the treatment group
- 5. Random Assignment vs. Random Sampling