Poisoning by the pesticide DDT causes tremors and convulsions. In a study of DDT poisoning, researchers fed several rats a measured amount of DDT. They then measured electrical characteristics of the rats' nervous systems that might explain how DDT poisoning causes tremors. One important variable was the "absolutely refractory period," the time required for a nerve to recover after a stimulus. This period varies normally. Measurements on four rats gave the data below (in milliseconds). 1.6 1.7 1.8 1.9 Find the mean refractory period x and the standard error of the mean. Give a 90% confidence interval for the mean "absolutely refractory period" for all rats of this strain when subjected to the same treatment. Suppose that the mean "absolutely refractory period" for un-poisoned rats is known to be 1.3 milliseconds. DDT poisoning should slow nerve recovery and so increase this period. Do the data in the previous exercise give good evidence for this supposition? State H_0and H_a and do a t-test. What do you conclude from the test?