The third method of data collection is measurement where standardized instruments are used to measure variables. For example, scientific instruments such as thermometer, barometer, weighing scale are instruments for physical measures of temperature, pressure and weight respectively. Precision measure depends on refinement of the instrument. A standard instrument should be used and preferably: the same instrument should be used: it is for measuring all subjects.
What is the difference between "measurement" and "assessment"? These two terms are sometimes used synonymously. But there is a difference between the two. In a simple way it can be said that measurement means to ascertain the dimensions, quantity or capacity of objects or events, whereas assessment can be defined as a systematic procedure of collecting quantitative as well as qualitative data to describe behaviour or characteristics. For example 8 rating scale to survey job satisfaction of nurses or survey learning needs of nurses qualifies as assessment where as a knowledge test on AIDS is used to measure knowledge of AIDS in students.
In measurement, numeral are assigned to objects or events according to rule to give a quantitative meaning. A numeral is a symbol of the form 1, 2, 3 and so on or I, II, III and so on. When quantitative meaning is given to the numerals, these become numbers. Hence, number is a numeral that has been given a quantitative meaning. It is easy to understand measurements used in natural sciences, which measure, length, weight and volume. Accurate scales are available for these. Measurements used in psychological, sociological, educational characteristics or nursing care standards are difficult to understand unless assignment of numerals to objects or events are made according to some accepted rules set up on rational or theoretical basis. For these measurements scales are prepared, like aptitude scale, nursing standard scale, etc.