贴一下我的解答：
Data. The data consist of samples from population A with x ̅_A=10.3750, s_A=1.3960 and n_A=12, and samples from population B with x ̅_B=12.2083, s_B=1.4493 and n_A=12.
Assumptions. The statistics were computed from two independent samples that behave as simple random samples from two populations of infants.
Hypotheses.
H_0:μ_A-μ_B=0
H_A:μ_A-μ_B≠0
Test statistic. Though the sample size are not large enough, we choose the z-statistics to test the hypothesis. Since the population variances are unknown, we will use the sample variances in the calculation of the test statistic. The test statistic is
z=(〖(x ̅〗_A-x ̅_B)-〖(μ_A-μ_B)〗_0)/√((s_A^2)/n_A +(s_B^2)/n_B )
Distribution of test statistic. If the null hypothesis is true, the test statistic is distributed approximately as the standard normal with μ=0.
Decision rule. Let α=.05. The critical value of the test statistic is 1.96. Our decision rule, then, is reject H_0 if the computed t is z>1.96 or z<-1.96.
Calculation of test statistic.
z=((10.3750-12.2083)-0)/√((1.3960)^2/12+(1.4493)^2/12)=-3.16
Statistical decision. Reject H_0 since-3.16<-1.96.
Conclusion. We can conclude that two populations of infants differ with respect to mean age at which they walked alone.
p value. The exact p value is 0.0008.
