Statistical and Practical Significance (4 of 4)
Next section: Type I and II errors
The 95% confidence interval on the difference between means is:
-5.8 ≤ μcolor - μblack & white ≤ -0.9
and the 99% interval is:
-6.6 ≤ μcolor - μblack & white ≤ -0.1
Therefore, despite the finding of a "more significant"
difference between means, the experimenter can be even more certain that
the color displays are only slightly better than the black and white displays.
The second experiment shows conclusively that the difference is less than
10 seconds.
This example was used to illustrate the following points:
(1) an effect that is statistically significant is not necessarily large
enough to be of practical significance and (2) the smaller of two effects
can be "more
significant" than the larger. Be careful how you interpret findings
reported in the media. If you read that a particular diet lowered cholesterol
significantly, this does not necessarily mean that the diet lowered cholesterol
enough to be of any health value. It means that the effect on cholesterol
in the population is greater than zero.
Next section: Type I and II errors