discusses Evidence based practice and what constitutes diagnostic evidence.

1. EBP LECTURE 3
Part 3

2. WHAT CONSTITUTES
EVIDENCE ABOUT PROGNOSIS?

3. Introduction
• Often our patients ask questions about their condition(s) such as
when or whether or how much it will improve?
• These are questions about prognosis. The PT should be skilled
enough to make an accurate prognoses? But how?
• For the how, the PT can obtain information about prognosis from
clinical observation and from clinical research.

4. CLINICAL OBSERVATION
• Clinical observation is one source of information, regarding prognosis.
• Experience clinicians over the years accumulate enough observation of
patients with similar conditions, thus are able to make an accurate
prognosis.
• Some can even distil specific information about a condition, stating an
accurate outcome.
• Astute PTs, may be able to see patterns in the characteristics of patients
who have good and who do not have good outcomes-able to recognize
prognostic factors.

5. Limitations in Clinical observations
1. Often interested in long term effects but patients do not come for
a long term follow-up.
2. Follow up is on subset of patients, not all patients. Subset does
not represent the whole population.
3. To observe accurate estimates of prognosis, PTs have to see and
observe several hundred patients of similar condition, which is
not very common.

6. Clinical Research
• To generate good information about prognosis, researchers must identify a group
of people with the condition of interest and see how those people’s condition
changes over time.
• Such studies are called longitudinal studies term ‘longitudinal’ implies that
observations on any one subject are made at more than one point in time.
• Types are:
1. Cohort Studies
2. Case-Control Studies

7. COHORT STUDIES:
• Longitudinal study that involves observing representative samples of people with
specific characteristics.
• There is no experimental manipulation involved; we investigate exposures among
study participants (at one point in time or over time) and observe their outcomes
either at the same point in time or sometime later on.
• Cohort simply refers to a group of people with some shared characteristics, such as
a shared diagnosis.
• Provide information about prognosis and how to refine prognosis.
Types Retrospective and Prospective Cohort Studies

10. Advantages of cohort studies
1. The time sequence of events can be assessed.
2. They can provide information on a wide range of disease outcomes.
3. The incidence/risk of disease can be measured directly.
4. Very detailed information on exposure to a wide range of factors can be collected.
5. They can be used to study exposure to factors that are rare.
6. Exposure can be measured at a number of time points in each study, so that changes in
exposure over time can be studied.
7. There is reduced recall and selection bias compared with case–control studies

11. Disadvantages of cohort studies
1. In general, a cohort study follows individuals for long periods of time, and it is therefore
costly to perform. (costly)
2. Where the outcome of interest is rare, a very large sample size is required.
3. As follow-up increases, there is often increased loss of patients as they migrate or leave
the study, leading to biased results. (increased loss to follow up)
4. Maintaining consistency of measurements and outcomes become difficult. Furthermore,
individuals may modify their behaviour after an initial interview. (Hawthorne effect)
5. It is possible that disease outcomes and their probabilities, or the aetiology of disease
itself, may change over time. (change in disease or cause)

13. CASE-CONTROL STUDY DESIGN: A retrospective epidemiologic research design
used to evaluate the relationship between a potential exposure (e.g., risk factor) and an outcome
(e.g., disease or disorder); two groups of subjects—one of which has the outcome (the case)
and one that does not (the control)—are compared to determine which group has a greater
proportion of individuals with the exposure.
For example, a group of athletes with repeated ACL tears may be studied in comparison with a
group of athletes without ACL tears. The outcome of interest—repeated ACL tears—has
already occurred, and the researchers are trying to identify the factors that contributed to these
repeated injuries.
• In a case control study, a group without the tears but comprising those who play the same
sport and who are of the same gender and age range would be compared on all factors
thought to contribute to the risk of tears.

15. ADVANTAGES OF CASE–CONTROL STUDIES
1. They are generally relatively quick, cheap and easy to perform.
2. They are particularly suitable for rare diseases.
3. A wide range of risk factors can be investigated in each study.
4. There is no loss to follow-up.
DISADVANTAGES OF CASE–CONTROL STUDIES
1. Recall bias
2. Causation cannot be inferred if onset precedes exposure to risk factors
3. Not suitable when exposure to risk factors are rare.

16. Homework
1. The Framingham Heart Study as an example of a cohort study
2. State the difference between a risk ratio and an odds ratio.
To be written in practical copy provided by INU. Will be checked at the time of viva
voce.