The alpha value is the probability of making a Type I Error. In a Hypothesis Test a Type I error occurs when statistically unlikely test results lead to the incorrect conclusion that the null hypothesis should be rejected. The alpha value is conventionally designated by the symbol alpha (α ).
The alpha value is also called the 'level of significance' and is selected based on the importance of the test, a value of 0.05 would be common, with 0.01 for tests that are critical, and even 0.001 in some cases.
A hypothesis test involves comparing the calculated p-value with the previously agreed alpha value. If the p-value is less than the alpha value the alternative hypothesis will be accepted.