p = Probability of obtaining teststatistic this extreme, given that H_{0} is true. Inference Rule: Reject H_{0} if teststatistic is "large" enough. THEREFORE: If we choose to reject H_{0} when p < 0.05, then the chance of having a False Alarm is no more than 0.05 (1 time in 20). When p < 0.05, we commonly say that the effect (in the case of the ttest, the difference between the groups on the dependent variable) is statistically significant. 

Note: It should be clear that p is defined relative to only half of the table of possible outcomes: when H_{0} is actually true. By concentrating on p, our primary concern is avoiding false alarms. The other side of the table, where H_{0} is actually false, concerns our ability to avoid missing an actual effect, i.e. the power of the test.