13.47 It has been reported that 18.3% of all U.S. households were heated by electricity in 1980, compared to 27.4% in 1995 and 31.5% in 2005. At the 0.05 level, and assuming a sample size of 1000 U.S. households for each year, test whether the population percentages could be equal for these years. Source: Statistical Abstract of the United States 1996, p. 722; Statistical Abstract of the United States 2009, p. 610.