http://www.ck12.org Chapter 6. Planning and Conducting an Experiment or Study
of the same subject, it is called arepeated measures design.Each runner would use shoeAand shoeBfor equal
periods of time and then the wear of the shoes for each individual would be compared. Randomization still could
be important. Let’s say that we have each runner use each shoe type for a period of 3 months. It is possible that the
weather during those three months could influence that amount of wear on the shoe. To minimize this, we would
randomly assign half the subjects shoeA, with the other half receiving shoeBand then switch after the first 3 months.
Lesson Summary
The important elements of astatistical experimentare randomness, imposed treatments, and replication. These
elements are the only effective method for establishing meaningful cause and effect relationships. An experiment
attempts to isolate, orcontrolother potential variables to may contribute to changes in the response variable. If
these other variables are known quantities but are difficult, or impossible, to distinguish from the other explanatory
variables, they are calledconfounding variables.If there is an additional explanatory variable affecting the response
variable that was not considered in an experiment, it is called alurking variable.Atreatmentis the term used to
refer to a condition imposed on the subjects in an experiment. An experiment will have at least two treatments. When
trying to test the effectiveness of a particular treatment, it is often effective to withhold applying that treatment to
a group of randomly chosen subjects. This is called acontrol group. If the subjects are aware of the conditions
of their treatment, they may have preconceived expectations that could affect the outcome. Especially in medical
experiments, the psychological effect of believing you are receiving a potentially effective treatment can lead to
different results. This phenomenon is called theplacebo effect. When the participants in a clinical trial are led to
believe they are receiving the new treatment, when in fact they are not, it is called aplacebo.If the participants are
not aware of the treatment they are receiving, it is called ablind experiment.When neither the participant nor the
researcher are aware of which subjects are receiving the treatment and which subjects are receiving a placebo, it is
called adouble-blind experiment.
Blockingis a technique used to control the potential confounding of variables. It is similar to the idea of stratification
in sampling. In arandomized block design,the researcher creates blocks of subjects that exhibit similar traits
which might cause different responses to the treatment and then randomly assigns the different treatments within
each block. Amatched pairs designis a special type of design when there are two treatments. The researcher
creates blocks of size two on some similar characteristic and then randomly assigns one subject from each pair to
each treatment.Repeated measures designsare a special matched pairs experiment in which each subject becomes
it’s own matched pair by applying both treatments and comparing the results.
Points to Consider
- What are some other ways that researchers design more complicated experiments?
- When one treatment seems to result in a notable difference, how do we know if that difference is statistically
significant? - How can the selection of samples for an experiment affect the validity of the conclusions?
Review Questions
- As part of an effort to study the effect of intelligence on survival mechanisms, scientists recently compared a
group of fruit flies intentionally bred for intelligence along with the same species of ordinary flies. When re-
leased together in an environment with high competition for food, the ordinary flies survived by a significantly