Critical t values in R

Question

I need to determine the critical t values for one-sided tails of 75  and 99   for 40 degrees of freedom   The following is code for a two-sided 99  critical t values   qt 0 01  40    but how can I determine for a one-sided critical t value

User · Accepted Answer

The code you posted gives the critical value for a one-sided test (Hence the answer to you question is simply:

abs(qt(0.25, 40)) # 75% confidence, 1 sided (same as qt(0.75, 40))
abs(qt(0.01, 40)) # 99% confidence, 1 sided (same as qt(0.99, 40))

Note that the t-distribution is symmetric. For a 2-sided test (say with 99% confidence) you can use the critical value

abs(qt(0.01/2, 40)) # 99% confidence, 2 sided

User · Answer

Extending  Ryogi answer above  you can take advantage of the lower tail parameter like so   qt 0 25 2  40  lower tail   FALSE    75  confidence  qt 0 01 2  40  lower tail   FALSE    99  confidence

User · Answer

Josh s comments are spot on   If you are not super familiar with critical values I d suggest playing with qt  reading the manual   qt  in conjunction with looking at a look up table  LINK    When I first moved from SPSS to R I created a function that made critical t value look up pretty easy  I d never use this now as it takes too much time and with the p values that are generally provided in the output it s a moot point    Here s the code for that   critical t  lt - function        cat   n    bEnter Alpha Level    n       alpha lt -scan n 1 what   double 0  quiet T      cat   n    b1 Tailed or 2 Tailed  nEnter either 1 or 2    n       tt  lt - scan n 1 what   double 0  quiet T      cat   n    bEnter Number of Observations    n       n  lt - scan n 1 what   double 0  quiet T      cat   n nCritical Value    qt 1- alpha tt   n-2     n      critical t

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