Design
A multiple-probe design was considered most appropriate for the present study.
This design permits the establishment of causality between the instructional program and
the subjects’ responses (Kazdin, 1992; Tawney & Gast, 1984). With a multiple-probe
design baseline data is collected simultaneously and continuously across 3 or more
subjects. Initially, a stable level is established for each subject and then the intervention is
applied sequentially to each subject. The subjects not immediately targeted are monitored
using probe sessions. The intervention is only applied once a previously set criterion level
is attained by the previous subject on the instructional material. In the present study the
criterion level was set at 90%. According to Tawney and Gast (1984), two conditions are
necessary to ensure validity with such a design. In the first place, all subjects must be
independent of each other so that one does not learn from the other before all have had an
opportunity to receive the instructional program. In the present study each subject was in
a different math group and, therefore, subjects did not interact during the math periods.
Second, subjects must be similar in terms of the problem area to be addressed. In the
present study all subjects were approximately the same age and had been diagnosed as
having a learning disability.