WASHINGTON - More than one in eight households have cell phones but lack traditional landline telephones, according to a federal study released Monday that tracks the country's growing dependence on wireless phones.The data, reported twice a year, suggested that the number of households relying solely on cell phones may be growing more slowly than it had in the past. But the researchers said the slowdown might be due to changes in their survey, including altering the order of some questions and some of the wording."We don't know how much reflects reality and how much reflects changes in the questionnaire," said …