Tertium quid refers to which phenomenon?

• A. A measurement error issue
• B. The third-variable problem where a third variable influences both variables
• C. A mediation effect
• D. The primary cause of spurious correlation

Answer: (B)

Tertium quid is a third variable that influences both variables in a relationship, producing a spurious association. This third-variable problem means that what looks like a link between two things may actually be driven by something else that affects both. For example, temperature can increase both ice cream sales and drowning incidents; the apparent connection between ice cream and drowning weakens when you account for temperature. In research, spotting this third variable helps you consider potential confounders and decide whether to control for them, such as with stratification or partial correlations. Mediation is a different idea, describing a pathway through which one variable affects another via a mediator, while measurement error is a separate type of distortion.