Well.. I do not think I can ever go back to that seat in the library to study again.. many people who are going to take the Stats exam tonight with me are dying.. cos they are trying to memorize this definition in the textbook.. (Seriously i do not think this can be ever called a definition lah, but the author put it in the definition box..)

Here it goes, take a deep breath before you start:

If the null hypothesis is true, you can make a type 1 error, but you cannot make a type 2 error; if the alternative hypothesis is true, you can make a type 2 error, but you cannot make a type 1 error. Therefore it only makes sense to talk about the probability of type 1 error given that the null hypothesis is true and the probability of making a type 2 error given that the alternative hypothesis is true. When the null hypothesis is not true, a type 1 error cannot be made; when the null hypothesis is true, the probability of a type 1 error, the level of significance are equivalent. When the alternative hypothesis is true, the probability of making the correct decision is called the power of the test.

hahah it is so funny to see everyone struggling to recite this paragraph, lol..