When one attempts to empirically explain such variables in terms of other variables and/or their own prior values, one uses time series or regression methods in which variables are indexed with a subscript indicating the time period in which the observation occurred. For example, y<sub>t</sub> might refer to the value of income observed in unspecified time period t, y<sub>3</sub> to the value of income observed in the third time period, etc. | When one attempts to empirically explain such variables in terms of other variables and/or their own prior values, one uses time series or regression methods in which variables are indexed with a subscript indicating the time period in which the observation occurred. For example, y<sub>t</sub> might refer to the value of income observed in unspecified time period t, y<sub>3</sub> to the value of income observed in the third time period, etc. |