level of measurement

n. The form and accuracy of observations. Observations can be recorded in nominal, ordinal, interval, and ratio scales so that a different set of operations can be performed on each. Thus taking the average of names or the standard deviation of ranks is not meaningful but is often essential for understanding interval or ratio data. Similarly the precision with which a set of categories fits the data or the accuracy with which an instrument measures a ratio variable also affects the usefulness of the data.