"lower.tail" in R I usually get confused on how to use the argument in "pt" function and similar function. Here I will focus on t-distribution. I will utilize Minitab for graphical presentation. A:- lower.tail is FALSE In R The code is > pt(q = -2.262, df = 9, lower.tail = FALSE) The output [1] 0.9749936 In Minitab This is shown as in the graph from Minitab. So when FASLE is chosen, the calculation will give the area after the critical value. B:- "lower.tail" is TRUE In R the code is pt(q = -2.262, df = 9, lower.tail = TRUE) the output is [1] 0.02500642 In Minitab When TRUE is used, it orders R to compute the area before the critical value.
To compute the "P" between two cut offs in t-distribution (two points) in R. The example uses the d.f. = 9 i.e. n = 10 , first quantile -2.262, the second quantile is 2.262. I used Minitab to give a graphical representation of that as below:- The code in R to use is as follow:- pt(q = 2.262, df = 9, lower.tail = TRUE) - pt(q = -2.262, df = 9, lower.tail = TRUE) I recomend to play a little with above code, to find out the argument "lower.tail" , when it is TRUE and FALSE. The output from R is :- [1] 0.9499872 Which when rounded, it will be 0.95 as Minitab. P.S d.f. ; degree of freedom.