Chapter 6: Appraising Prognostic Research Studies

Pre-Test

1. How does the appraisal process change when considering prognostic research in comparison to intervention research?

2. What are typical study designs for prognostic research that one is likely to find in the literature?

3. What is the flaw in the following statement about a regression analysis? It appears that subject weight, previous injuries, and type of profession were all causes of low back pain.

Introduction

Prognostic Questions in the Clinic

Patients, families, and physical therapists have many questions about prognosis. Prognostic questions may be about the impact of a disease or event on a patient's long-term outcome. For example, a patient may ask, "Will I be able to ski after back surgery?" or "When can I expect to go back to work?" Prognostic questions may also guide discharge planning. You may ask, "What is the prognosis for my patient to return home versus a rehabilitation facility at discharge?" or "What are the odds that this intervention will benefit my patient's independent ambulation?" Many factors influence the answers to prognostic questions such as the severity of the patient's problem, gender, age, home environment, and co-morbidities. Valid prognostic studies can assist in answering these types of questions, and they assist in weighing the various factors that may contribute to specific outcomes. The prognostic literature has specific types of designs and statistical analyses. This chapter begins with design descriptions for prognostics studies, as these will be different from the types of designs used for intervention studies (Fig. 6.1).

Research Designs That are Specific to Prognostic Studies

The same principles for searching the literature described in Chapter 2 also apply to searching the prognostic literature. However, the research designs most typically applied to prognostic questions are observational studies that used associations between or among variables. These typically include cohort and case control designs (Fig. 6.2), which can be either longitudinal (subjects are followed over time) or cross-sectional (subject data are collected at one point in time).

Cohort Studies

In a cohort study, a group of subjects who are likely to develop a certain condition or outcome is followed into the future (prospectively) for a sufficient length of time to observe if the subjects develop the condition. Cohort studies can provide data concerning the timing of the development of the outcome within the group and assist in defining possible causal factors in developing the condition. The factors, or risks, for a particular outcome are identified, and the effect is observed.

For example: