Kritieke Waarden

Confidence limits at a confidence level—say at 95%—cover 95% of the area under the curve.

Remaining 5% of the area, which is designated by α, is distributed equally on either tail of the data distribution. In the case of 95% level, it would be 2.5%.

In other words, α/2 = 0.025.

Recall that calculating estimates for population parameters require z scores obtained using the z distribution.

Such a z score calculated at the right tail of the distribution—that is—at positive α/2 is known as the critical value. It is denoted as z_α/2.

A critical z value for a given confidence level is a fixed value. It does not change over any number of samples or for a statistic.

To calculate the critical z value for any confidence level, look for 1−α/2 value in the z table.

For the 95% level, look for 0.975, not 0.95, to note the value of 1.96.

Similarly, for 90% and 99% confidence levels, the critical z values are 1.645 and 2.575, respectively.