1. (Easiest Way) Use known values for z alpha/2.
You don’t actually have to look up z alpha/2 in a z-table every time. For most statistical
tests, you’ll probably be using one of four confidence intervals (90%, 95%, 98% and 99%).
The z alpha/2 for each confidence level is always the same:
2. Use a Z-Table
Step 1: Find the alpha level. If you are given the alpha level in the question (for example, an
alpha level of 10%), skip to step 2. Subtract your confidence level from 100%. For example, if
you have a 95 percent confidence level, then 100% – 95% = 5%.
Step 2: Divide the amount you found in Step 1 by 2 to get the alpha level for a two-tailed
test:
.50/2 = 2.5 percent.
Step 3: Subtract Step 2 from 50%:
50% – 2.5% = 47.5%
Step 4: Convert Step 3 to a decimal and find that area in the center of the z-table.
The closest z-score to 47.5 percent (.475) is at z=1.96.
Critical values for t (two-tailed) Use these for the calculation of confidence intervals. For
example, use the 0.05 column for the 95% confidence interval.
df 0.10 0.05 0.025 0.01
2 2.9200 4.3027 6.2054 9.9250
3 2.3534 3.1824 4.1765 5.8408
4 2.1318 2.7765 3.4954 4.6041
5 2.0150 2.5706 3.1634 4.0321
6 1.9432 2.4469 2.9687 3.7074
7 1.8946 2.3646 2.8412 3.4995
8 1.8595 2.3060 2.7515 3.3554
9 1.8331 2.2622 2.6850 3.2498
10 1.8125 2.2281 2.6338 3.1693
11 1.7959 2.2010 2.5931 3.1058
12 1.7823 2.1788 2.5600 3.0545
13 1.7709 2.1604 2.5326 3.0123
14 1.7613 2.1448 2.5096 2.9768
15 1.7531 2.1315 2.4899 2.9467
16 1.7459 2.1199 2.4729 2.9208
17 1.7396 2.1098 2.4581 2.8982
18 1.7341 2.1009 2.4450 2.8784
19 1.7291 2.0930 2.4334 2.8609
20 1.7247 2.0860 2.4231 2.8453
21 1.7207 2.0796 2.4138 2.8314
22 1.7171 2.0739 2.4055 2.8188
23 1.7139 2.0687 2.3979 2.8073
24 1.7109 2.0639 2.3910 2.7970
25 1.7081 2.0595 2.3846 2.7874
26 1.7056 2.0555 2.3788 2.7787
27 1.7033 2.0518 2.3734 2.7707
28 1.7011 2.0484 2.3685 2.7633
29 1.6991 2.0452 2.3638 2.7564
30 1.6973 2.0423 2.3596 2.7500
31 1.6955 2.0395 2.3556 2.7440
32 1.6939 2.0369 2.3518 2.7385
33 1.6924 2.0345 2.3483 2.7333
34 1.6909 2.0322 2.3451 2.7284
35 1.6896 2.0301 2.3420 2.7238
36 1.6883 2.0281 2.3391 2.7195
37 1.6871 2.0262 2.3363 2.7154
38 1.6860 2.0244 2.3337 2.7116
39 1.6849 2.0227 2.3313 2.7079
40 1.6839 2.0211 2.3289 2.7045
41 1.6829 2.0195 2.3267 2.7012
42 1.6820 2.0181 2.3246 2.6981
43 1.6811 2.0167 2.3226 2.6951
44 1.6802 2.0154 2.3207 2.6923
45 1.6794 2.0141 2.3189 2.6896
46 1.6787 2.0129 2.3172 2.6870
47 1.6779 2.0117 2.3155 2.6846
48 1.6772 2.0106 2.3139 2.6822
49 1.6766 2.0096 2.3124 2.6800
50 1.6759 2.0086 2.3109 2.6778
51 1.6753 2.0076 2.3095 2.6757
52 1.6747 2.0066 2.3082 2.6737
53 1.6741 2.0057 2.3069 2.6718
54 1.6736 2.0049 2.3056 2.6700
55 1.6730 2.0040 2.3044 2.6682
56 1.6725 2.0032 2.3033 2.6665
57 1.6720 2.0025 2.3022 2.6649
58 1.6716 2.0017 2.3011 2.6633
59 1.6711 2.0010 2.3000 2.6618
60 1.6706 2.0003 2.2990 2.6603
61 1.6702 1.9996 2.2981 2.6589
62 1.6698 1.9990 2.2971 2.6575
63 1.6694 1.9983 2.2962 2.6561
64 1.6690 1.9977 2.2954 2.6549
65 1.6686 1.9971 2.2945 2.6536
66 1.6683 1.9966 2.2937 2.6524
67 1.6679 1.9960 2.2929 2.6512
68 1.6676 1.9955 2.2921 2.6501
69 1.6672 1.9949 2.2914 2.6490
70 1.6669 1.9944 2.2906 2.6479
71 1.6666 1.9939 2.2899 2.6469
72 1.6663 1.9935 2.2892 2.6458
73 1.6660 1.9930 2.2886 2.6449
74 1.6657 1.9925 2.2879 2.6439
75 1.6654 1.9921 2.2873 2.6430
76 1.6652 1.9917 2.2867 2.6421
77 1.6649 1.9913 2.2861 2.6412
78 1.6646 1.9908 2.2855 2.6403
79 1.6644 1.9905 2.2849 2.6395
80 1.6641 1.9901 2.2844 2.6387
81 1.6639 1.9897 2.2838 2.6379
82 1.6636 1.9893 2.2833 2.6371
83 1.6634 1.9890 2.2828 2.6364
84 1.6632 1.9886 2.2823 2.6356
85 1.6630 1.9883 2.2818 2.6349
86 1.6628 1.9879 2.2813 2.6342
87 1.6626 1.9876 2.2809 2.6335
88 1.6624 1.9873 2.2804 2.6329
89 1.6622 1.9870 2.2800 2.6322
90 1.6620 1.9867 2.2795 2.6316
91 1.6618 1.9864 2.2791 2.6309
92 1.6616 1.9861 2.2787 2.6303
93 1.6614 1.9858 2.2783 2.6297
94 1.6612 1.9855 2.2779 2.6291
95 1.6611 1.9852 2.2775 2.6286
96 1.6609 1.9850 2.2771 2.6280
97 1.6607 1.9847 2.2767 2.6275
98 1.6606 1.9845 2.2764 2.6269
99 1.6604 1.9842 2.2760 2.6264
100 1.6602 1.9840 2.2757 2.6259
