Social Research Methods: Qualitative and Quantitative Approaches

EXPERIMENTAL RESEARCH

are variations of the classical experimental design.
Some have randomization but lack a pretest, some
use more than two groups, and others substitute
many observations of one group over time for a
control group. In general, the researcher has less
control over the independent variable than in the
classical design (see Table 1).
Two-Group Posttest-Only Design. This design is
identical to the static group comparison with one
exception: You randomly assign. It has all parts of
the classical design except a pretest. Random as-
signment reduces the chance that the groups dif-
fered before the treatment, but without a pretest,
you cannot be as certain that the groups began the
study at the same level on the dependent variable.
In a study using a two-group posttest-only de-
sign with random assignment, Rind and Strohmetz
(1999) examined restaurant tips. The treatment in-
volved messages about an upcoming special written
on the back of customers’ checks. The participants
were eighty-one dining parties eating at an upscale
restaurant in New Jersey. The treatment was whether

a female server wrote a message about an upcom-

ing restaurant special on the back of a check and

the dependent variable was the size of the tip. The

researchers gave a server with two years’ experience

a randomly shuffled stack of cards. One-half said

No Message and one-half said Message. Just before

she gave a customer his or her check, she randomly

pulled a card from her pocket. If it said Message,

she wrote about an upcoming special on the back of

the customer’s check. If it said No Message, she

wrote nothing. The experimenters recorded the

amount of the tip and the number of people at the

table. They instructed the server to act the same

toward all customers. The results showed that

higher tips came from customers who received the

message about upcoming specials.

Interrupted Time Series. In an interrupted time-

series design, you measure the dependent variable

on one group over time using many multiple de-

pendent variable measures before (prettests) and

after a treatment (posttests).

Equivalent Time Series. An equivalent time-series

designis a one-group design similar to the inter-

rupted time series design. It extends over a time pe-

riod, but instead of a single treatment, the equivalent

time series design has the same treatment multiple

times. Like the interrupted time series design,

we measure the dependent variable several times

before and after the treatments. The study on alco-

hol sales and suicide rates (Example Box 3,

TABLE 1 A Comparison of the Classical Experimental Design

DESIGN

RANDOM

ASSIGNMENT PRETEST POSTTEST

CONTROL

GROUP

EXPERIMENTAL

GROUP

Classical Ye s Ye s Ye s Ye s Ye s

One-shot case study No No Ye s No Ye s

One-group pretest/postest No Ye s Ye s No Ye s

Static group comparison No No Ye s Ye s Ye s

Two-group posttest only Ye s No Ye s Ye s Ye s

Time-series designs No Ye s Ye s No Ye s

Equivalent time-series design An experimental

plan with several repeated pretests, posttests, and treat-

ments for one group often over a period of time.

Interrupted time-series design An experimental

plan in which the dependent variable is measured pe-

riodically across many time points and the treatment

occurs in the midst of such measures, often only once.