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. if we are to test whether the population percentage could be equal for these years at the 0.05 level, and assuming a sample size of 1000 u. s. households for each year, what is the calculated chi square value