# 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 .

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 .

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

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).

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