The type of statistical test which is valid to use in different situations will depend upon the type of measurement scale that was incorporated during the study. The four types of statistical tests are described below.
Nominal scales classifies persons or objects into two or more categories. Members of a category have a common set of characteristics, and each member may only belong to one category. True catagories are those to which the member naturally falls, such as gender (male vs. female), level of formal education (high school, bachlor's, master's, doctorate), or preschool attendance (did attend preschool or did not attend preschool).
Artificial categories are those to which the researcher places the members, such as strength of IQ (high versus medium versus low), family income (above the average versus below the average), or learning style (field independent versus field dependent).
Ordinal scales both classify subjects and rank them in terms of how they possess the characteristic of interest. Members are placed in terms of highest to lowest, or most to least. Students may be ranked by height, weight, or IQ scores. Ordinal scales do not, however, state how much difference there is between the ranks. A person who is 6 feet tall may be first, 5 foot 11 inches may be second, and 5 foot 3 inches third. In other words, the difference in rank between rank 2 and rank 3 may not be the same as the difference between rank 5 and rank 6.
Interval scales have all the same characteristics of nominal and ordinal scales, but have the additional characteristic of having predetermined equal intervals. In other words, the difference between a score 1 and a score of 5 is the same as the difference between a score of 41 and a score of 45.
Interval scales normally have an arbitrary minimum and maximum point. A score of zero in an IQ test does not represent an absence of intelligence, nor does an IQ score of 200 represent perfect intelligence.
Ratio scales have all the advantages of the other three scales, but also have the advantage of having a meaningful, true zero point. Height, weight, time, speed, and distance are examples of ratio scales. Measurements made with ratio scales can be added, subtracted, multiplied, and divided. For example, we can say that a person who runs a mile in 5 minutes is twice as fast as a person who runs the mile in 10 minutes. Interval scales (such as IQ scores) can not be used in the same fashion. Because ratio scales are often used in physical measurements (but not psychological measures) ratio scales are not often employed in educational tests.
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