Index measures the US unemployment rate on a monthly basis. The unemployment rate represents the number unemployed persons as a percent of the labor force. Each month approximately 60,000 sample households are interviewed (a portion is done via telephone) during the week that includes the 12th of the month. History is stored independently for monthly, quarterly and yearly frequencies. HP values for these frequencies may not match each other. Note that when only one frequency is stored in history, for the other frequencies, history is derived from that of the stored frequency. For example, HP W, HP M, HP Q and HP Y will derive data from HP D when only daily history is stored, HP Q and HP Y will derive data from HP M when only monthly history is stored. As another example, if both daily and monthly are stored, HP W will derive data from HP D, while HP Q and HP Y will derive data from HP M and so on.