Adding load is very quantifiable; it makes it easy to measure progress.
A timely post. I was talking to Aaron Thigpen and Carl Valle about this topic earlier this week.
In addition to load, another parameter that is quantifiable is speed. Some coaches select the load based on a percentage of body weight and other coaches select the load based on percentage of speed reduction. Do you measure and log load/speed progression, and if so, how would you characterize the typical progression? In particular, are athletes generally able to increase the load and/or speed over multiple sessions, and does this correlate with their unloaded speed?