Frequency tables often have open-ended classes, such as the accompanying table (based on data for Ontario from Statistics Canada), which summarizes annual household incomes for a sample of 5197 families of 3-4 persons. Formula 2-2 cannot be directly applied because we can't determine a class midpoint for the class "more than 80,000." Calculate the mean by assuming this class is really
(a) 80,000 up to 100,000,
(b) 80,000 up to 110,000, and
(c) 80,000 up to 120,000. What can you conclude?

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Annual Household Income (S) L Frequency 0 up to 20,000 20,000 up to 40,000 40,000 up to 60,000 60,000 up to 80,000 More than 80,000 404 1081 982 1310 1420