Measurements differ according to the degree of precision with which they measure a particular characteristic scales. Various scales are employed in the field of measurement. Stevens has recognised four types of scales.
1. Nominal scales
2. Ordinal scales
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3. Interval scales
4. Ratio Scales.
1. Nominal Scales:
This is the most limited type of measurement involving classification of data e.g.: Numericals or numbers are assigned such as group 1 or group 2. The concept of continuous does not apply to a nominal scale. It only helps to classify or identify objects or individuals, but tells us nothing about their quality.
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2. Ordinal Scale:
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This involves a ranking, arranging or sequencing of data. For e.g.: school children are put in 1,2,3,4 orders for height marks, etc. Rank order gives us a serial position in the group, but it does not provide an exact measurement. For e.g.: Salesmen can be ranked for their experience or advertisements for their zest or popularity. Ranks cannot be added or subtracted as we add weight or inches because a person’s / object’s rank is always relative to the ranks of other members of the group.
It does not provide an absolute measure. E.g.: A student getting the 5th rank and a student getting 10th rank is not comparable to those getting 15th rank and 20th rank. Compulation of the median, percentiles and rank – order correlation can be done from this scale.
3. Interval Scale:
In interval scale scores are expressed in equal units. Standard psychological tests are usually interval scales, as they have equal units on equal steps, but they do not possess a true zero. In comparison physical scales for length, weight and volume have equal units and are taken from a zero point. For e.g. a length of 50 cms, is twice the length of 25 cms.
But we cannot add the units of a psychological scale and say, an IQ of 100 is twice the ability of an IQ of 50. This scale is an advancement from the previous too, as it not only gives quantitative but qualitative differences too, between individuals.
4. Ratio Scales:
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When measures are expressed in equal units and are also taken from a true zero, they constitute ratio scales, e.g.: c.g.s. scales (centimeters, grams, and seconds) found in physical sciences. Here, measures from a c.g.s. scale can be added / subtracted and a score of 20 inches in twice a score of 10 inches. Ratio scales are rarely encountered in behavioural sciences i.e. Mental / Social Variables and traits arc rarely expressed in ratio scales. The measurement of pitch and loudness can be expressed in ratio scales.