Analogy

Each student in a class was given a bin of wooden blocks and asked to sort them based on their characteristics. It would be most consistent to sort them if they were all cubes, all the same color, and only differed on size. Each student would likely sort their bin the same. However, if the blocks were different colors, shapes, and sizes, students would sort them differently. Some may sort by size, others by color, and others by shape. Here, the characteristics of a block are underlying traits or dimensions that are being measured. Unidimensionality would be when the blocks only differ on size – size is the single dimension being measured.

Diagram

The diagram is showing unidimensionality. Specifically, it is illustrating that the shapes are only different on one dimension – size.  This is similar to a scale that is measuring only one construct.  For example, a spelling test is only measuring spelling – that is the single construct being measured.  In this diagram, the only trait being ‘measured’ is the size of the blocks.

Example

A common example of unidimensional vs. multidimensional comes from math.  If you are given a problem such as 2+2, it is testing your simple addition skills.  That is the one construct, and no others.  However, if the problem is instead “Juan has two apples.  Susie comes up and gives them two more apples.  How many apples does Juan have now?”, not only is it testing simple addition, but now there is the added dimension of reading.  A weak reader may answer the word problem incorrectly, but the ‘numbers-only’ problem correctly.  If we only test on one dimension (math), we clearly know that the one thing we are looking at is their simple addition skills.  This way, if they get it wrong, we know it is their simple addition skills that need reinforcing.

Plain Language Definition

A measure is unidimensional when it only measures one thing.

Technical Definition

A measure that is unidimensional consists of items that tap into only one dimension; the items share a common primary construct.