Productivity is the efficiency of production of goods or services expressed by some measure. Measurements of productivity are often expressed as a ratio of an aggregate output to a single input or an aggregate input used in a production process, i.e. output per unit of input, typically over a specific period of time. The most common example is the (aggregate) labour productivity measure, one example of which is GDP per worker. There are many different definitions of productivity and the choice among them depends on the purpose of the productivity measurement and data availability. The key source of difference between various productivity measures is also usually related to how the outputs and the inputs are aggregated to obtain such a ratio-type measure of productivity.