This standardization procedure allows us to measure all normal distributions in terms of a common unit – standard deviation units – regardless of the units in which the variable is initially measured. It is analogous to expressing the price of various goods from different countries in terms of a common currency. We may have a whole list of prices some of which are expressed in US dollars, some in British pounds, others in Swiss francs. But if we convert all the prices into a single currency a comparison can be made. Similarly, a distribution may be expressed in terms of centimetres, or crime rates, or births per thousand. But expressing these various distribution according to standard deviation units giver a common scale of measurement.