P=0.49903〉α,按正态性假定进行统计分析。 方差齐性检验程序和结果如下:(a=0.10) sdtest x, by(group) 方差齐性检验 Variance ratio test Mean Std. Err. Std. Dev 5%Conf. Interval] 对照组 6.968235262574396536.4357997.500201 实验组|10 8.196 38405211.2144797.3272149.064786 combined I 7.582.26054561.1651957.0366728.127328 Ho:sd(对照组)=sd(实验组) F(9, 9) observed = F obs 0.375 F(9, 9) lower tail = FL =F ob F(9, 9)upper tail FU =1/F obs 2.665 Ha: sd(1)< sd(2) Ha:sd(1)!=sd(2) Ha: sd(1)>sd(2) P<Fobs=0.0802P<FL+P>FU=0.1604 Fobs=0.9198 方差齐性检验的P=0.1604,可以按方差齐性的假定作进一步统计分析。 例7-5对例7-1资料,试比较两种饮水小鼠肝脏平均锌含量有无差别? 相应的 Stata程序为 ttest x, by (group) 两独立样本t检验 输出结果为 Two-sample t test with equal variances Mean Std. Err. Std. Dev. [95% Conf. Interval]P=0.49903>，按正态性假定进行统计分析。 方差齐性检验程序和结果如下： ( 0.10)  = sdtest x,by(group) 方差齐性检验 Variance ratio test ------------------------------------------------------------------------------ Group | Obs Mean Std. Err. Std. Dev. [95% Conf. Interval] ---------+-------------------------------------------------------------------- 对照组 | 10 6.968 .2352625 .7439653 6.435799 7.500201 实验组 | 10 8.196 .3840521 1.214479 7.327214 9.064786 ---------+-------------------------------------------------------------------- combined | 20 7.582 .2605456 1.165195 7.036672 8.127328 ------------------------------------------------------------------------------ Ho: sd(对照组) = sd(实验组) F(9,9) observed = F_obs = 0.375 F(9,9) lower tail = F_L = F_obs = 0.375 F(9,9) upper tail = F_U = 1/F_obs = 2.665 Ha: sd(1) < sd(2) Ha: sd(1) != sd(2) Ha: sd(1) > sd(2) P < F_obs = 0.0802 P < F_L + P > F_U = 0.1604 P > F_obs = 0.9198 方差齐性检验的 P= 0.1604，可以按方差齐性的假定作进一步统计分析。 例 7-5 对例 7-1 资料，试比较两种饮水小鼠肝脏平均锌含量有无差别？ 相应的 Stata 程序为 ttest x,by(group) 两独立样本 t 检验 输出结果为: Two-sample t test with equal variances ------------------------------------------------------------------------------ Group | Obs Mean Std. Err. Std. Dev. [95% Conf. Interval] ---------+--------------------------------------------------------------------