The spread of the t distribution is a bit greater than that of the standard norm Table c gives critical values for t distributions distribution The t distributions have more Each row in the table contains critical values probability in the tails and less in the for one of the t distributions; the degrees of center than does the standard normal freedom appear at the left of the row This is true because substituting the estimates s for the fixed parameter o By looking down any column, you can check introduces more variation into the statistic that the t critical values approach the normal values as the degrees of freedom increase As the degrees of freedom k increase, the t(k) density curve approaches the N(o, 1) curve ever more closely. This happens Table t ecause s estimates o more accurately distribution as the sample size increases. So using s critical in place of o causes little extra variation when the sample is large 温需11 21 • The spread of the t distribution is a bit greater than that of the standard normal distribution. The t distributions have more probability in the tails and less in the center than does the standard normal. This is true because substituting the estimates s for the fixed parameter introduces more variation into the statistic. σ 22 • As the degrees of freedom k increase, the t(k) density curve approaches the N(0,1) curve ever more closely. This happens because s estimates more accurately as the sample size increases. So using s in place of causes little extra variation when the sample is large. σ σ 12 23 Table C gives critical values for t distributions. Each row in the table contains critical values for one of the t distributions; the degrees of freedom appear at the left of the row. By looking down any column, you can check that the t critical values approach the normal values as the degrees of freedom increase. 24 Table C t distribution critical values