Although commonly expressed as the ratio of the output produced to the input consumed, numerous manufacturing productivity measures have been developed depending upon the different types of outputs and resource inputs. Single-factor (or partial) productivity (for example Gold 1971 and Roll and Sachish 1984) is the ratio of output to a single (or partial) physical input (i.e. labour, capital, material or energy). Each single-factor productivity measure is usually named after its input type: labour, capital, material or energy productivity.