# Student t-values

The values for "t" for different sample sizes and different confidence intervals are given in the Table 1.  Please note that degrees of freedom corresponds to number of cases minus the number of parameters estimated from the sample.  In most control charts, the degrees of freedom equals to one minus number of cases.

 Degrees of Freedom Confidence Interval 90% 95% 99% 1 6.31 12.71 63.66 2 2.92 4.3 9.93 3 2.35 3.18 5.84 4 2.13 2.78 4.6 5 2.02 2.57 4.03 6 1.94 2.45 3.71 7 1.89 2.37 3.5 8 1.86 2.31 3.36 9 1.83 2.26 3.25 10 1.81 2.23 3.17 11 1.8 2.2 3.11 12 1.78 2.18 3.06 13 1.77 2.16 3.01 14 1.76 2.14 2.98 15 1.75 2.13 2.95 16 1.75 2.12 2.92 17 1.74 2.11 2.9 18 1.73 2.1 2.88 19 1.73 2.09 2.86 20 1.72 2.09 2.85 21 1.72 2.08 2.83 22 1.72 2.07 2.82 23 1.71 2.07 2.82 24 1.71 2.06 2.8 25 1.71 2.06 2.79 26 1.71 2.06 2.78 27 1.7 2.05 2.77 28 1.7 2.05 2.76 29 1.7 2.05 2.76 30 1.7 2.04 2.75 40 1.68 2.02 2.7 60 1.67 2 2.66 120 1.66 1.98 2.62 ¥ 1.65 1.96 2.58 Table 1:  Student t DistributionObtained from The Really Easy Statistics Site, Produced by Jim Deacon, Biology Teaching Organization, University of Edinburgh.  For details of the original site see http://helios.bto.ed.ac.uk/bto/statistics/table1.html

For example, the t-value for 95% confidence interval, for a sample size of 31 has 30 degrees of freedom and from the table it is 2.04.

# Estimating Values Not Given in the Table

To approximate the t-values for sample size not given in the table, find the nearest two values in the table and select the larger value.

This page is part of the course on Quality/Process Improvement.  It was created by Farrokh Alemi, Ph.D. and last revised on 05/03/2015