Overview of single subject designs

Author: Dr Simon Moss

Suppose a researchers wanted to examine whether body odor affects the extent to which they are perceived as attractive, but because of limited resources can only examine one participant. In these instances, the researcher must select one of three options. First, an ABAB design could be applied (see Interrupted time series for statistical techniques that could examine the data in this context):

• That is, the researcher could repeat the treatment and control conditions on many occasions.
• To illustrate, on every second day they could ask participants to apply dencorub.
• On every other day, they could ask participants to refrain from dencorub.
• Hence, any effects of dencorub cannot be ascribed to time.

Unfortunately, to undertake this approach, researchers must withdraw the treatment during every second session, which is sometimes unethical. Hence, a multiple baseline design could be applied instead. That is:

• The researcher could apply two treatments, which are hypothesised to influence separate dependent measures
• To illustrate, on the first day, no treatment is applied
• On the second day, dencorub, which is hypothesised to enhance attraction, is applied
• On the third day, both dencorub and vegemite, which is hypothesised to enhance mood, is applied
• If the hypotheses are supported, attraction should be elevated on the second and third day. Mood should be elevated on the third day only.
• This rationale can be extended to more than two treatments

Unfortunately, this approach can be applied only when the researcher wants to explore several treatments that influence distinct outcomes. If this condition is not fulfilled, an incremental approach can be adopted instead. For example:

• On the first day, no treatment is applied
• On the second day, 5 mg of dencorub is applied
• On the third day, 10 mg of dencorub is applied
• On the fourth day, 15 mg of dencorub is applied
• If the hypothesis is true, attraction should increase gradually over time.

These single-subject designs can be applied when the project comprises more than one participant. For example, this design is applicable when the project comprises five participants, but each individual effectively represents a separate study.