This is a difficult unit. Not only is it filled with
vocabulary, but it has some subtle concepts. Our approach -- which we
recommend -- is to focus on experiments vs. observational studies, and
discuss the basic forms of surveys. In Unit 15, after studying
probability and getting experience with estimation, we'll return and
discuss some of the more subtle refinements of experimental design,
such as blocking and stratifying.
• Experimental Design
Designing an experiment is complex and probably won't be understood
until after completing the unit. Mastery won't be achieved until later
sections that provide a more complete context are covered. We will
return to this topic later in the course.
Our primary goal is to determine the effect of a treatment variable on
a response variable. We want the treatment groups to be as similar as
possible to one another in all respects except for the values of the
treatment variable. Only if this is achieved can we conclude that
differences in the response variable between the groups is due to the
differences in the treatment variable.
When comparing groups we are looking for differences in their "typical"
response (usually), and this is complicated by the variation in the
response variable within each group. Variation has various sources, and
blocking -- which divides the units into homogeneous groups --
minimizes variation due to one particular source. (Look back to the
Demonstration in Unit 1, in which children were paired, a form of
The only way to make inferences about a population based on a sample
taken from that population is to make sure your sample is
representative of the population. The only way to do this is to take a
random sample from the population. There are various techniques to take
a random sample, and only a few of them are included in the AP
curriculum. For example, the most basic is the simple random sample
(SRS) in which items are selected from the population at random but
without replacement. A refinement of this technique is stratification,
in which the population is divided into stratas, and a SRS is taken
from within each strata.
• The Big 3
Randomization, Repetition, and Control!