The single subject time series design (SSTSD) is a form of a case report that uses a more formal process to track the response of a single patient over time. One of the challenges physicians face is the knowledge that it is never really possible to know, for a given single patient, that what we have done to the patient has resulted in the response we see. There can be many reasons a patient responds after therapy, such as natural history, the intervention, placebo effect, and so on. However, the SSTSD provides us with a better window into the usefulness of our intervention, since it tracks a single person over time.
In an SSTSD, you would initially establish a baseline on your patient over a period of at least 3 visits. Here, you would collect data on the patient’s status. For example, perhaps we were working with a patient who has chronic low back pain, and our outcomes measures include an Oswestry Disability Index as well as a Visual Analogue Scale. Taking these three measures gives us a stable sense of the patient’s status. Then, we begin treatment, and during the period of treatment we continue to take these same outcome measurements. We do so for the same length of time as we did for the baseline period. We might see a change in scores when we do this, a reduction in both disability and pain. This can help establish a relation between our intervention and the patient response. Now, let’s add one more period of measurement, a follow-up period where we cease therapy again, and instead measure the outcomes over a similar period of time. If we see the scores rise or return to where they were before we started treatment, this is even better data demonstrating the impact that our therapy has. This is known as an ABA design for a SSTSD.
Conditions ripe for study as SSTSDs need to be stable and chronic; acute conditions vary too quickly for a physician or researcher to be able to establish a meaningful baseline for comparison, because as you can see, the patient acts as his or her own control during the treatment and follow-up time. These kinds of studies are useful for helping a patient to understand change over time, especially for conditions where change in made only incrementally and slowly (for example, frozen should or adhesive capsulitis).
Variations in this design exist. For example, an ABA design can be repeated over time as an ABAB design, also known as a withdrawal design since it withdraws and then restores therapy several times. There can be an ABAC design, where C represents a second, alternative intervention. We can also consider a multiple baseline design, in which at least 3 individuals are tracked like this, but with the caveat that we do not begin treatment on the second patient until after we see a response in the first, and we do not begin treatment on the third patient until we see a response in the second. The differing baselines here are a means to reduce effects of confounding influences. There is also a simultaneous baseline design in which we do the same as in a multiple baseline approach but gather all our patients at the same time; in a multiple baseline approach it is not necessary to do so.
This methodology can be useful in a practice setting, and can be a means for a chiropractor to collect data for future publication. It also is a means to strengthen the results drawn from case reports, which tend to fall lower on an evidence hierarchy. I do not neglect case reports, though; they are often the papers clinicians turn to first, and they can offer direction for challenging patients.