Organic phosphorous insecticides are very stable chemically and are known to collect in the soil and water and eventually to enter the food chain of human beings. In a study made in an agricultural region in the Orient, the milk of 40 nursing mothers was examined and found to have an average of 4.2 ppm of organic phosphorous insecticides. The sample standard deviation was 1.2 ppm.
a. Place a two-sided 99% confidence interval on the mean level of these compounds in mothers' milk in the region.
b. Place a one-sided 99% confidence limit on the worst the mean contamination might be.