In the Journal of Earthquake Engineering (Nov. 2004), a team of civil and environmental engineers studied the ground motion characteristics of 15 earthquakes that occurred around the world. Three (of many) variables measured on each earthquake were the type of ground motion (short, long, or forward directive), earthquake magnitude (Richter scale) and peak ground acceleration (feet per second). One of the goals of the study was to estimate the inelastic spectra of any ground motion cycle.

a. Identify the experimental units for this study.

b. Do the data for the 15 earthquakes represent a population or a sample? Explain.