Chapter 11.2, Problem 10AYU

Secchi Disk A Secchi disk is an 8-inch-diameter weighted disk that is painted black and white and attached to a rope. The disk is lowered into water and the depth (in inches) at which it is no longer visible is recorded. The measurement is an indication of water clarity. An environmental biologist interested in determining whether the water clarity of the lake at Joliet Junior College is improving takes measurements at the same location on eight dates during the course of a year and repeats the measurements on the same dates five years later. She obtains the following results:

1. a. Why is it important to take the measurements on the same date?
2. b. Does the evidence suggest that the clarity of the lake is improving at the α = 0.05 level of significance? Note: A normal probability plot and boxplot of the data indicate that the differences are approximately normally distributed with no outliers.
3. c. Draw a boxplot of the differenced data. Does this visual evidence support the results obtained in part (b)?