Stiffness measures the rigidity of the asphalt surface, and provides an output in CMV. While it can have a correlation to pavement core density values, this measurement is open to interpretation.

AASHTO R47-23 Reducing Samples of Hot Mix Asphalt (HMA) to Testing Size Exempt if qualified in Asphalt Module, taken within one year (doesn't apply for written exam) AASHTO T310-22 In-Place Density ...