Select menu: Spread | Factor | Reference Level
For a factor, a particular level is used in the parameterization of regression models as the base level. The effects of the other factor levels are then defined as differences from this base or reference level. The option <Default – First Level> specifies the first level or the factor, and if the factor levels are reordered, the new first level will become the reference level. If a particular level has been chosen as a reference level, the spreadsheet will attempt to always make this the reference level on any operations that reorder the factor levels.