https://www.isaacdhale.com/presentations/POL%20106%20-%20The%20Invisible%20Primary%20(Winter%202021).pdf

This is a powerpoint presentation from Professor Isaac Hale at The University of California Davis which, beginning on slide 20, clearly defines the invisible primary, discusses the role of parties in the process, analyzes the pre-primary phases of the 2008 and 2016 elections, and displays a series of relevant graphics and statistics.