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Three Kinds Of Measurement

Measurement by classification into categories:

Qualitative variables are involved. These are variables that differ in kind not amount. Gender is an example of a qualitative variable.
Classification is used for placing individuals, objects or events into categories.
Two or more categories are used for classification.

An Example of Measurement by Classification:

366 Voters = 200 Democrat, 100 Republican, 66 Independent

Measurement by Ranking:

Describes the standing or rank order of an individual in a particular group.

Examples of Measurement by Ranking:

Class rank describes a student's standing relative to classmates.
The rank order of sports teams in the standings (i.e. First, Second, Third).
The rank order of children by height ranging from the "tallest" to "shortest".

Measurement by ranking tells nothing about the actual amounts of a variable.
From rank alone the researcher does not know that the tallest child in a class is
63 inches tall. Nor does ranking provide information about the differences in height
between the tallest and the shortest child in the class.

Measurement by Amount:

Applying the use of some kind of measuring instrument to "measure" the variable.
Measuring instruments are calibrated in a particular unit of measurement (i.e. inches, degrees, meters, light years, grams, rotations per minute, etc.).
Findings are reported by stating the amount of the variable in terms of the number of units on the measuring instrument.

Examples of Measurement by Amount:

Using a thermometer to measure temperature.
Using an IQ test to measure cognitive ability.
Length of an object measured with a ruler.

Measurements can be continuous or discrete:

Continuous measures can hypothetically take any value between the lowest and highest possible. Suppose the actual length of an object's is 8.765432100987654433...mm. However, if this object is measured with a ruler accurate only to the nearest 1/2 millimeter, the object would be reported as being 9 mm long. However, its actual length is somewhere in the continuous interval between 8.5 and 9.0 mm.
Discrete measures can only take specific values. Families may have 2 children or 3 children. But they can not have 2.67 or 3.55 children. Discrete variables are usually measured by counting.