Page 73 - Statistics for Dummies

Chapter 4: Tools of the Trade
Treatment group versus control group
Most experiments try to determine whether some type of experimental treat-
ment (or important factor) has a significant effect on an outcome. For example,
does zinc help to reduce the length of a cold? Subjects who are chosen to par-
ticipate in the experiment are typically divided into two groups: a treatment
group and a control group. (More than one treatment group is possible.)
✓ The treatment group consists of participants who receive the experimen-
tal treatment whose effect is being studied (in this case, zinc tablets).
✓ The control group consists of participants who do not receive the experi-
mental treatment being studied. Instead, they get a placebo (a fake treat-
ment; for example, a sugar pill); a standard, nonexperimental treatment
(such as vitamin C, in the zinc study); or no treatment at all, depending
on the situation.
In the end, the responses of those in the treatment group are compared with
the responses from the control group to look for differences that are statisti- 57
cally significant (unlikely to have occurred just by chance).
Placebo
A placebo is a fake treatment, such as a sugar pill. Placebos are given to the
control group to account for a psychological phenomenon called the pla-
cebo effect, in which patients receiving a fake treatment still report having a
response, as if it were the real treatment. For example, after taking a sugar
pill a patient experiencing the placebo effect might say, “Yes, I feel better
already,” or “Wow, I am starting to feel a bit dizzy.” By measuring the placebo
effect in the control group, you can tease out what portion of the reports from
the treatment group were real and what portion were likely due to the placebo
effect. (Experimenters assume that the placebo effect affects both the treat-
ment and control groups.)
Blind and double-blind
A blind experiment is one in which the subjects who are participating in the
study are not aware of whether they’re in the treatment group or the control
group. In the zinc example, the vitamin C tablets and the zinc tablets would
be made to look exactly alike and patients would not be told which type of pill
they were taking. A blind experiment attempts to control for bias on the part
of the participants.
A double-blind experiment controls for potential bias on the part of both the
patients and the researchers. Neither the patients nor the researchers collect-
ing the data know which subjects received the treatment and which didn’t. So
who does know what’s going on as far as who gets what treatment? Typically
a third party (someone not otherwise involved in the experiment) puts

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