On July 23, 2006, the Houston Chronicle published an article entitled "Reading: First-grade standard too tough for many". The article claimed in part that "more students (across Texas) are having to repeat first grade. Experts attribute the increase partially to an increase in poverty." The article presents data for each of 61 Texas counties on Y = Percentage of students repeating first grade x = Percentage of low-income students for both 2004-2005 and 1994-1995. The data can be found on the book web site in the file HoustonChronicle.csv. Use analysis of covariance to decide whether:

a. An increase in the percentage of low income students is associated with an increase in the percentage of students repeating first grade.

b. There has been an increase in the percentage of students repeating first grade between 1994-1995 and 2004-2005.

c. Any association between the percentage of students repeating first grade and the percentage of low-income students differs between 1994-1995 and 2004-2005.