During the past three-year period, the average monthly wireless bill has increased a full $11: from $55 in 2004 to $66 in 2006. the percentage of wireless customers using text messaging on a regular basis has risen from 19% in 2005 to 25% in 2006 – a 32% increase. J.D. Power and Associates measures customer satisfaction based on 42 specific service-related measures grouped into six key factors that impact overall wireless carrier performance. They are (in order of importance): call performance and reliability (26%); customer service (17%); service plan options (17%); brand image (14%); cost of service (14%); and billing (12%). Carriers are ranked across six regions in the United States: Northeast, Mid-Atlantic, Southeast, North Central, Southwest and West.