Orthoteers homepage Advertise on Orthoteers
Orthoteers Junior Orthoteers Orthopaedic Biomechanics Orthopaedic World Literature Society Educational Resources Image Gallery About Orthoteers Orthoteers Members search

Statistics examples

CONFIDENCE INTERVALS

Example 1:

A drug lowered diastolic BP by a mean of 8mmHg from 100 to 92mmHg (p= 0.02; 95% CI = 2-14mmHg)

  • The above p value suggests a significant drop when the CI suggests it is not (i.e. if we tested 100 people 95 would show a drop of between 2-14mmHg).
  • Clearly a drop of 2mmHg is not clinically significant, therefore the clinical significance of the results are shown by a CI (plus medical knowledge)

Example 2: 

Comparing two groups 

(from - Hyperstat )

When you see a confidence interval in a published medical report, you should look for two things. First does the interval contain a value that implies no change or no effect ? For example, with a confidence interval for a difference look to see whether that interval includes zero. With a confidence interval for a ratio, look to see whether that interval contains one.

Here's an example of a confidence interval that contains the null value. The interval shown below implies no statistically significant change .

Figure 2.1

Here's an example of a confidence interval that excludes the null value. If we assume that larger implies better, then the interval shown below would imply a statistically significant improvement .

Figure 2.2 (1222 bytes)

Here's a different example of a confidence interval that excludes the null value. The interval shown below implies a statistically significant decline .

Figure 2.3 (1214 bytes)

Practical significance

You should also see whether the confidence interval lies partly or entirely within a range of clinical indifference . Clinical indifference represents values of such a trivial size that you would not want to change your current practice. For example, you would not recommend a special diet that showed a one year weight loss of only five pounds. You would not order a diagnostic test that had a predictive value of less than 50%.

Clinical indifference is a medical judgement, and not a statistical judgement . It depends on your knowledge of the range of possible treatments, their costs, and their side effects. As statistician, I can only speculate on what a range of clinical indifference is. I do want to emphasize, however, that if a confidence interval is contained entirely within your range of clinical indifference , then you have clear and convincing evidence to keep doing things the same way (see below).

Figure 2.4 (1558 bytes)



This is a preview of the site content. To view the full text for this site, you need to log in.
If you are having problems logging in, please refer to the login help page.


© 2005-2007 Orthoteers.co.uk - last updated by Len Funk on 06 April 2005Medical Merketing and SEO by Blue Medical 
Biomet supporting orthoteersThe British Orthopedic Association supporting OrthoteersOrthoteers is a non-profit educational resource. Click here for more details
About
Anatomy Atlas
Basic sciences
Classifications
Clinical Examination
Courses
Elbow
Extras
Foot and Ankle
FRCS(Tr&Orth) Exam
Glossary
Hand & Wrist
Help
Hip & Pelvis
Humerus
Imaging in Orthopaedics
Knee
Log Book
MCQ
News
Newsletters
Orthopaedic infections & Microbiology
Orthopaedic pathology
Paediatric orthopaedics
Perioperative issues
Rehabilitation
Shoulder
Spine
Statistics
Surgical approaches
Trauma
Home
Hide Menu