In 2011, the Behavioral Risk Factor Surveillance System (BRFSS) began sampling cell phone as well as landline telephone numbers and implemented an updated weighting methodology for annual state data files. Due to these changes, data from 2010 and earlier cannot be combined with data from 2011 forward.
The Wisconsin Statistics on Health (WISH) data query system now includes combined landline and cell phone data for the years 2011-2014, as well as landline-only data for earlier years.
BRFSS landline protocol, which includes random selection of one adult to interview in each eligible household reached at sampled numbers, excludes adults living in institutions and most group quarters. Cell phone interviews are conducted with people aged 18 or older reached at randomly selected numbers. Landline and cell phone sampling is done for all states by a sampling contractor employed by the Centers for Disease Control and Prevention (CDC), based on sample design criteria submitted by states.
Additional information is available from CDC.