http://www.ck12.org Chapter 6. Planning and Conducting an Experiment or Study
6.2 Experimental Design
Learning Objectives
- Identify the important characteristics of an experiment.
- Distinguish between confounding and lurking variables.
- Use a random number generator to randomly assign experimental units to treatment groups.
- Identify experimental situations in which blocking is necessary or appropriate and create a blocking scheme
for such experiments. - Identify experimental situations in which a matched pairs design is necessary or appropriate and explain how
such a design could be implemented. - Identify the reasons for and the advantages of blind experiments.
- Distinguish between correlation and causation.
Introduction
A recent study published by the Royal Society of Britain^1 concluded that there is a relationship between the
nutritional habits of mothers around the time of conception and the gender of their child. The study found that
women who ate more calories and had a higher intake of essential nutrients and vitamins were more likely to
conceive sons. As we learned in the first chapter, this study provides useful evidence of an association between
these two variables, but it is an observational study. It is possible that there is another variable that is actually
responsible for the gender differences observed. In order to be able to convincingly conclude that there is a cause
and effect relationship between a mother’s diet and the gender of her child, we must perform a controlled statistical
experiment. This lesson will cover the basic elements of designing a proper statistical experiment.
Confounding and Lurking Variables
In an observational study such as the Royal Society’s connecting gender and a mother’s diet, it is possible that there
is a third variable that was not observed that is causing a change in both the explanatory and response variables.
A variable that is not included in a study but may still have an effect on the other variables involved is called a
lurking variable. For example, perhaps the mother’s exercise habits caused both her increased consumption of
calories and her increased likelihood of having a male child. A slightly different type of additional variable is called
a confounding variable. Confounding variablesare those that are observed but it cannot be distinguished which