28.5 About finding \(P\)-values
As demonstrated in Sect. 27.5.1, often \(P\)-values can be approximated by using the the 68–95–99.7 rule and using a diagram of a normal distribution. The \(P\)-value is the area more extreme than the calculated \(t\)-score; the 68–95–99.7 rule can be used to approximate this tail area.
For two-tailed tests, the \(P\)-value is the combined area in the left and right tails. For one-tailed tests, the \(P\)-value is the area in just the left or right tail.
More accurate estimates of the \(P\)-value can be found using \(z\)-tables, though we do not demonstrate this in this book. Even more precise estimates of \(P\)-values can be found using specially-prepared \(t\)-tables. Again, we do not do so in this book.
For more precise \(P\)-values, we will take the \(P\)-values from software output.
When using software to obtain \(P\)-values, be sure to check if the software reports one- or two-tailed \(P\)-values.For example, some software (such as SPSS) always reports two-tailed \(P\)-values.