HP3804 NTU Psychology Statistics: Module Review (Defining a Construct)

Note: this post is part of a series of posts regarding HP3804 (Psychological Testing)

What is a construct? A seemingly simple question that we brushed over back in HP1100 — but now, the question haunts us again.

When we first started out in Psychology — we understood a construct as a ‘thing/concept/variable” of interest. Now that we have more familiarity as to common constructs in Psychology — it’s time to delve deeper into this definition.

Construct — One Thing or Multiple Things?

We all know what extroversion is. Extroversion is a construct — a core aspect of your personality. But what about personality itself? Personality itself can also be viewed as a construct — whereby you are interested in examining how personality predicts other variables (e.g. workplace success).

Both extroversion and personality are constructs — yet it is clear that they are not on the same “level” (extroversion is a subset of personality!). Thus, to distinguish between these types of constructs — we are going to need more precise vocabulary.

Unidimensional vs Multidimensional Constructs

Personality would be what we term as a multidimensional construct — in that within that construct, there are numerous subdomains. In order to measure personality, you would first have to break it down into these different subdomains, then quantify the scores within those subdomains. Importantly, mulitdimensional constructs are made up of multiple unmidimensional constructs — which refer to fundamentally different, singular things (see below).

Extroversion however, would be what is known as a unidimensional construct. It is the most “fundamental” already — is it is a singular thing (no more subdomains!). While there can be different aspects of extroversion (“enjoying parties, having many friends, talking loudly?) — these are merely different manifestations of the same underlying singular construct of extroversion.

Relationship between Dimensions in a Multi-Dimensional Construct

So — a “fundamental” construct is what we term as a dimension (I will be using this dimensionality terminology from now on — instead of the imprecise term of “construct”). A multidimensional construct has multiple dimensions— and each of these dimensions represents fundamentally different “components” of the multi-dimensional construct.

Take for instance personality. We all know that according to the Big Five Model, there are 5 components of personality — namely Extroversion, Agreeableness, Conscientiousness, Neuroticism, Openess to Experience. These components are generally seen as independent of each other (while I’ve heard of some research papers examining the correlation between certain domains, I think it’s still quite safe to say that they are rather distinct from each other). Plotting them out on a graph (say extroversion & agreeableness), the axes would look like this:

Importantly, the axes are orthogonal (unrelated) to each other — and it is entirely possible to increase/decrease in agreeableness WITHOUT affecting your extroversion scores at all (as shown by the black x above).

However, dimensions of a multidimensional construct can also be related to each other — instead of being fundamentally disparate. An example would be say, verbal reasoning ability and abstract reasoning ability in Spearman’s Intelligence Model. In this case, the multidimensional construct of Intelligence has dimensions that are not strictly orthogonal to each other (increasing one usually is associated with an increase in another). Graphically, it looks like this:

Notice what happened? The axes are no longer perpendicular to each other! Indeed — just a simple rotation of the axes and we are able to elegantly illustrate the relationship between the factors well. Now, an increase in abstract reasoning ability would also cause an increase in verbal reasoning ability — highlighting that the 2 factors are correlated to some extent.

Side note: for multidimensional contructs with independent factors, interpreting the multidimensional construct as a whole usually does not make sense (what does it mean when you say someone is “higher in personality” than other person?) For multidimensional constructs with related factors however, interpreting the multidimensional construct still has value (one can be higher in general intelligence than another person).

Measuring a Dimension

Now that we have settled on the definitions of a dimension/construct, we can get on to measuring them. It is natural that when we measure things — we only want to measure one thing at a time. (imagine that when you stepped on your weighing scale to measure weight, the output you see is strictly your BMI (weight/height²). You can’t isolate it into the components at all!) As a result, the rule of thumb is that each measurement scale should only measure a single, unidimensional construct.

It is with this understanding that we can go on to the next segment of the course — on understanding reliability of a scale. Stay tuned!