Since it's a one-tailed test the whole level of significance is contained in the one tail. [If it were two-tails, it would be split half and half between the two tails.] On your TI-84 press 2ND, VARS, highlight invT <-- invT, not invZ because 25 is less than 30 area:0.05 df:24 <-- 1 less than n Paste invT(0.05,24) <-- what you should have on your screen press ENTER -1.710882023, which rounds to -1.711 The reason it's negative is because the calculator gives critical values for the left tail. Since your choices are all positive, they want a right tail, so by symmetry, make it +1.711. Edwin