Single Subject Designs

Overview Two basic approaches to research: group designs (between group comparisons) single subject designs (a.k.a. within subject designs, intra-subject designs, etc.) Single subject designs originated in behavioral psychology, with “operant conditioning” of rats

Single subject designs in Comm. Disorders limited sample size hard to obtain large samples with disordered or special populations small samples result in low statistical power when using between group designs sometimes the sample consists of only one or two clients need for extensive or repeated observations more time consuming to run subjects individually harder to do detailed, in-depth observations

The basic concept of single subject designs Each subject serves as his/her own treatment and control condition The subject alternates between the treatment and control conditions changes in the subject’s speech/language performance are compared across the various treatment and control conditions often, “longitudinal” or “time-series” designs are used (employing ongoing measurements at time 1, time 2, time 3, etc.)

Nomenclature for single subject designs Phases or stages of the experiment are typically designated by letters A phase: initial baseline performance; preliminary diagnosis or assessment before any treatment. B phase: treatment condition. therapy is initiated for a specified period of time. A phase (2nd): withdrawal phase. Therapy is withdrawn to see if the client’s speech/language performance decays. C phase: alternative treatment condition; a second type of therapy is introduced.

Types of Single Subject Designs Descriptive designs (not true experiments) A only design: or case study: observation only, no manipulation, no control. A client is observed, speech/language samples are taken, no therapy is provided. B only design: treatment or therapy only, no baseline measures, no control. A client receives treatment, but no speech/language assessment is performed prior to beginning treatment AB design: baseline measures are taken, followed by the introduction of therapy. However, control is lacking (improvements could be due to spontaneous recovery, history, maturation, etc.)

More Types of Single Subject Designs Quasi-experiments ABA design: a stable baseline is established, followed by the onset of therapy, followed by the withdrawal of therapy. Speech should improve during the treatment phase Speech should regress during the withdrawal phase A B A

More Types of Single Subject Designs Quasi-experiments ABAB design: a stable baseline is established, followed by the onset of therapy, followed by the withdrawal of therapy. Speech should improve during the treatment phase Speech should regress during the withdrawal phase Speech should improve again with the reintroduction of treatment A B A B

More complicated designs ABABA reversal: Instead of reintroducing the treatment during the second B phase, treatment is reversed. That is, incorrect speech/language is reinforced. ABACA design: Instead of the second B phase, an alternative treatment condition, C, is introduced. The researcher wants to know if speech/language improves more during the B or C phase. Note: many other combinations are possible

Matched pairs design Used when a researcher has 10-15 subjects more than enough subjects for a single-subject design, yet still too few for a true between-groups design Subjects are rank ordered on the basis of their assessments on some speech/language variable Example: 12 stutterers are rank ordered 1 through 12 according to stuttering severity. Subjects are then paired off with their most similar counterparts In order to get the pairing precise enough, a researcher may recruit partners outside of the study. Example: stutterers 1 and 2 form a matched pair, stutterers 3 and 4 form a matched pair…etc. Comparisons are then made between the performances of the matched pairs

Illustration of a matched pairs design 12 Stutterers are rank ordered according to stuttering severity Biff Rudolpho Rex Otto Fifi Zack Muffin Tad Dieter Lulu Raheem Felix Matched pairs Treatment Control Biff Rudolpho Otto Rex Fifi Zack Tad Muffin Dieter Lulu Felix Raheem

Matched groups design A “between groups” design, that avoids random assignment in favor of matching on specific, predefined criteria A matched groups design is similar to matched pairs. Instead of matching one individual with another, the two groups are matched with each other. Groups may be matched on multiple criteria (sex, age, language, etc.) The researcher compares the average performance of one group with the other, as opposed to the performance of one individual with his/her matched counterpart.