In addition to knowing how constructs are operationalized as variables, it is important to understand how such variables are classified and manipulated by researchers in their quest to empirical knowledge.
To that end, we describe five different functions of variables.
In fact the independent variable is what you (or nature) manipulates -- a treatment or program or cause. The dependent variable is what is affected by the independent variable -- your effects or outcomes.
A moderator variable is a special type of independent variable which you may select for study in order to investigate whether it modifies the relationship between the dependent and independent variables.
Example, sex in the study of the effect of instruction on students’ reading scores
It is virtually impossible to include all the potential variables in each study. As a result, the researcher must attempt to control, or neutralize, all other extraneous variables that are likely to have an effect on the relationship between the independent, dependent, and moderator variables.
Nominal scale 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. Other names: categorical, discontinuous, dichotomous (only two categories).
Ordinal variables allow us to rank order the items we measure in terms of which has less and which has more of the quality represented by the variable, but still they do not allow us to say "how much more.“
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.
Not only rank order the items that are measured, but also to quantify and compare the sizes of differences between them.
For example: students performance on a spelling test A score of 16 will be higher than 14 and lower than 18 and the difference between them is 2 points (equal intervals).
Interval scales normally have an arbitrary minimum and maximum point. A score of zero in a spelling test does not represent an absence of spelling knowledge, nor does a score of 20 represent perfect spelling knowledge.
V ery similar to interval scale; has all the properties of interval variables, it has absolute zero point. Height, weight, 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. Because ratio scales are often used in physical measurements (where absolute zero exists), they are not often employed in educational research and testing.