Relative Risk (RR) and Confidence Interval (CI).
Relative risks and confidence intervals are usually expressed numerically or in graphic form. I have made a graph which displays four different RRs and their CIs and will try to explain how to interpret them.
Group 1 has a RR of 1.5 and the CI is 1.1 to 2.5 which are generally shown in text form as RR 1.5 ( CI 1.1-2.5). This RR would be considered marginally significant since the lower range of the CI is just above the line for RR of 1 which is called the no effect level or line.
Remember if the range of the CI includes the no effect line of 1 the data are not statistically significant.
Group 2 also has a RR 1.5 (CI 0.8-2.5) but the lower range of its CI is includes the no effect value of 1.. Therefore it would not be considered statistically significant.
Group 3 has a RR 0.5 (CI 0.2-0.9) which is an example of a RR which is showing
a decreased risk. Since the upper range of the CI does not include the no effect level of 1 this RR would be judged to be statistically significant.
Group 4 has a RR 0.5 (CI 0.2-1.1) which is another example of an RR showing a decreased risk. However, in this case the upper range of the CI includes the no effect level of 1 and would not be considered statistically significant.
STATISTICAL SIGNIFICANCE DOES NOT MEAN THE DATA SHOW A SIGNIFICANT BIOLOGICAL EFFECT OR CAUSE
When two groups are judged to be statistically significant it just means that the two groups are probably not the same. Something is different,- but what?
Such differences could be due to chance as in studies with large confidence intervals which almost include the no effect level.
These types of studies attempt to show associations or correlations between some event and risk of some disorder or problem. Showing that the data are significant does not prove a cause and effect only the possibility of one.
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