Disclaimer: This article is primarily intended for an online group of post graduate students in Community Medicine that I am involved with. The group was created to provide supplemental instruction to members on topics of common interest. Instruction is in bite-sized portions, since all members are busy PG students. Conceptual understanding is emphasized. Membership to that (Whatsapp) group is through invitation only. However, others interested in participating in the discussions and related activities in Google classroom may indicate the same by sending me a message on Facebook.
In the last session we discussed scales of measurement, and identified the appropriate scale of measurement for some common variables. This article will build on that knowledge, and describe the categorical and continuous variables in greater detail.
The four scales of measurement (Nominal, Ordinal, Interval, Ratio) may be reduced to just two broad types of variables: categorical/ discrete; and continuous variables.
Discrete (Categorical) variables: These include the nominal scale and ordinal scale. These variables have mutually exclusive levels (membership in one level excludes membership in another). For instance, one cannot simultaneously have two different blood groups; or be simultaneously rich and poor based on economic criteria. Categorical variables may have only two levels (living status: alive/dead), or more than two levels (Socioeconomic status: Low/ Middle/ High). When a categorical variable has only two levels, it also called a binary variable.
Continuous variables: As the name suggests, these variables include interval and ratio scale, and consists of variables that have meaningful numerical values. (When one assigns a numeric code to a categorical variable, the numbers are not meaningful, since the assignment of numbers was arbitrary- taking mean of those numbers will not be meaningful [The variable Sex consists of three levels: Male (1), Female (2), and Others (3). Taking the mean of the numbers associated with each level (also known as the numeric code) will yield a value of 2. What is the meaning of this mean value? Nothing, since the numbers were arbitrarily assigned to each level. One could just as easily have assigned values of 111, 222, and 333 to each level.]. However, when the numbers are meaningful, the mean is also meaningful. [The height of a person is a meaningful entity, so the mean of height is also meaningful.]).
In the next article, we will discuss another way of classifying variables.
Link to article on Scales of Measurement: