Could you please tell me how can we know which percentage points should we take for our tests ? in the solutions for examples, why does he choose 0,5% for Q 12.20 and 0,1% for 12.21 ? How can we judge? Thanks a lot
You will be told at what level of significance (5% or 1%) you need to do a test or compute a confidence interval (like 95% or 99%). In case nothing is mentioned, normal convention is to do at 5% significance. In Q12.20/12.21, actually the notes tries to compare the critical value at the most extreme significance point (like 0.5% or 0.1%) that is available in the tables and conclude that as the computed test statistics value is so large that the null hypothesis can be rejected at all reasonable levels of significance. I will suggest have a look at some of the past exam papers. Even the ones in the Q&A bank or the X-series assignments will help you get an idea what is usually asked. In cases where they don't clearly state the level of significance, either you could do something like what is been done here or do it at the 5% significance level. Do of course clearly state what you are doing to gain full-marks. Hope this helps.
