The t-test gives you two statistics that you can use to conduct a test of the mean difference: a t-value and a p-value. The t-value is not very informative by itself, but it is used to determine the p-value. The p-value tells you just how likely it is that you would obtain your distribution of differences, with its particular mean and standard deviation, if H0 is true.

You must decide the p-value required to reject H0 before you conduct the test. The value that you choose as your criterion is called the a-level. If the p-value is less than or equal to your a-level, then you reject H0 and conclude that m is not equal to the reference value.

You can choose any a-level that is greater than 0% and less than 100%. The 0.05 a-level is commonly used.

For the heart rate data the t-value is 2.70, and the associated p-value is 0.017. A p-value of 0.017 indicates that there is only a 1.7% chance that you would have obtained your sample difference if m D was actually 0.