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Have you ever reassessed a patient by repeating measurements and wondered whether the change (either improvement or worsening) that may have occurred indicated real and important change or whether it was simply measurement error? More than likely, the smaller this change, the more you questioned whether it was real or important. Well, if you answered yes to the question, we are not surprised. Judging change is one of the most common dilemmas faced by physical therapists with practically every patient at some point during the plan of care. In Chapter 1 we stated, “The conscientious and judicious assessment of patients' outcomes is a complex clinical skill, much like patient assessment and treatment selection and delivery, and requires a conceptual framework and a specific body of knowledge.” Perhaps nowhere is the complexity of outcome assessment more evident than in understanding the interpretation of change scores. Key to this issue is the interpretation of threshold values of change supplied in different research papers and secondarily in many textbooks.
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When deciding whether a patient has improved (or worsened), a physical therapist judges whether the difference between current and previous assessments' values meets a predetermined threshold value, and this impression regarding whether change has occurred or not is recorded in the medical record. Historically, decisions concerning a patient's change status were guided by clinical experience alone. Today, study-based threshold values for many measures are available to augment clinical experience. Ideally, study-based estimates of change complement clinical experience by providing not only an evidence-based threshold value but also a representation of the confidence a physical therapist can have in making the correct decision when applying the threshold value.
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Figure 6-1 introduces the possible consequences faced by a physical therapist when deciding whether a patient has or has not improved. This figure reminds us that not all patients with change scores meeting the reported threshold value will be truly improved. Neither will all patients with change scores less than the threshold value be unimproved. Physical therapists can be confident in their decisions to the extent that the threshold value accurately classifies the change status of patients. For an outcome measure to be clinically useful, a physical therapist must know the threshold value for change and be confident that applying this value provides a good chance of correctly labeling a patient's true change status.
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In this chapter we describe three popular methods used by researchers to estimate threshold change values for individual patients. We also explain the interpretation of these estimates and compare the information offered by each method to the information required by physical therapists when making decisions about the change status of patients. In addition, we alert readers to three other critical points when ...