Since the sample mean results in a t-value of -3.039 which is below the critical t value of -2.776, we can conclude the test provides evidence that the top tube process change results in a lower weight with a 95% confidence. The test statistic is the calculated number of t values the sample mean $- \bar=-2.776-$ keeping in mind the t-distribution is symmetrical and we are interested in the lower tail region. The alternative hypothesis for a z-test may be: Or, if the test is to detect a shift higher or lower (one-sided) There are three choices depending if you want to check if the mean has changed from an expected value either higher or lower (two-sided). Next, specify the alternative hypothesis. If either assumption is not true the results of the t-test statistic may not be informative. Generally, when n > 25 the difference between the z and t-tests is very small. Note as the sample size goes up this becomes less of a concern due to the central limit theorem.