Course:  Data analysis: hypothesis testing

Section: 5.2 Test your understanding

Table 4 Calculating alpha levels

The calculation of alpha from z-scores for the two tailed examples are incorrect.
Since the alpha needs to be divided by two to calculate the z-score, then the final z-score needs to be multiplied by two to calculate the associated alpha. In the answers given the z-score has been divided by two and hence these values are 4 times smaller than they should be. I have checked this using available online calculators.

Answers should be ( in bold)
Test                        z-score   Alpha levels                         Given answer
One-tailed test     2.35        0.00938670553483856      ~0.01
Two-tailed test     1.96        0.049995790296441           ~0.01      ** Incorrect
One-tailed test     1.65        0.0494714680336481         ~0.05
Two-tailed test     1.29        0.197050658099496           ~0.05      ** Incorrect