In experimental research, we often **operationalize constructs** using **nominal variables** when they are actually **continuous variables**. We do this because it can be easier to examine the impact of different **levels** of the independent variable on the dependent variable when these **levels** are **discrete**. In the table below, we provide some examples where we may choose to use **discrete levels** where **continuous measurements** are more reflective of the **behaviour** of a given construct.

Construct | Discrete levels | Continuous measurement |

Age | Under 21 21 - 30 31 - 40 41 - 50 51 - 64 65 + |
Age is a continuous measurement from birth (i.e., 1 second old) to death (e.g., 80 years, 23 days, 5 hours, 2 minutes, 17 seconds). It is a continuous variable that can be measured with accuracy. |

Background music Loudness of music |
Low Medium High |
Sound pressure level (SPL) with a range from 0 dB to 120 dB (i.e., audible range of the human ear). |

Teacher ability Level of experience |
1 year 10 years 20 years |
Experience may be better evaluated in years since there is a large difference between 1 and 10 years. Also, it may be debatable whether 20 years of experience is twice as much experience as 10 years (in real terms). |

When we operationalize constructs using nominal variables when they are actually continuous variables, we cannot be sure that the same results would have been obtained if **more precise** measurements were taken that better reflect the **behaviour** of a given construct.

It is possible that the **treatment** can **sensitise** the participant to the construct that is being measured in such a way that the **factorial structure** of the construct **changes**, at least in terms of the way that the participant views such a construct. This is a threat to construct validity known as **treatment-sensitive factorial structure** (Heppner et al., 2008).

For example, we may view a construct as **one-dimensional** (e.g., employee stress/burnout or addiction). However, after having received the treatment, especially if such a treatment is an educational program of sorts that leads the participant to understand the construct better than they did originally (e.g., they may be able to distinguish between different aspects of employee stress/burnout, such as loss of desire, difficulty getting up in the morning, attitude towards co-workers, etc.). This can make a **one-dimensional** construct into a **multi-dimensional** construct, despite the fact that participants only have one way of responding (i.e., a total score for stress/burnout). This can happen when mono-measures are used.

There is a lot of **ambiguity**, not only in the way that constructs can be **operationally defined**, but also how different constructs **relate** to one another (e.g., how the construct, **anger**, relates to the construct, **depression**). What are the **boundaries** between these different constructs? Where does one construct start and the other end?

When different construct **overlap**, the results that we generate when measuring these construct can become **confounded**. We discuss the impact of **extraneous** and **confounding variables** in more detail in the article: Extraneous and confounding variables. In the article, Convergent and divergent validity, we discuss one of the ways of checking whether constructs have been confounded.

You can learn more about the different types/tests of validity that help to establish construct validity in the following articles: Content validity, Convergent and divergent validity, Criterion validity (concurrent and predictive validity) and Reliability in research.

Campbell, D. T., & Fiske, D. W. (1959). Convergent and discriminant validation by the multitrait-multimethod matrix. *Psychological Bulletin*, *56*: 81-105.

Cronbach, L. J., & Meehl, P. E. (1955). Construct validity in psychological tests. *Psychological Bulletin*, *52*: 281-302.

Goodwin, C. J. (2009). *Research in Psychology: Methods and Design*. John Wiley and Sons.

Messick, S. (1980). Test validity and the ethics of assessment. *American Psychologist*, *35*: 1012-1027.