Hypothesis Testing with One Sample
Read this section on the two types of errors in hypothesis testing and some examples of each.
Type II Error
A type II error occurs when the null hypothesis is false but erroneously fails to be rejected. It is failing to assert what is present, a miss. A type II error may be compared with a so-called false negative (where an actual "hit" was disregarded by the test and seen as a "miss") in a test checking for a single condition with a definitive result of true or false. A type II error is committed when we fail to believe a truth. In terms of folk tales, an investigator may fail to see the wolf ("failing to raise an alarm"). Again, 
no wolf.
The rate of the type II error is denoted by the Greek letter 
(beta) and related to the power of a test (which equals 
).