Rule sets. For example:
Growth is not just about the measurement, but the factor involves dedication and regimentary discipline. Some of us have harder to grow penises, while the rest may have an easier time. Since you are the guru, you have to make a rule set.
This includes time period of the challenge, complexity of the routines and progressive gains, measurement of using photo evidence and not just words, and verification of tools being used.
What this will set up is a data collection challenge, providing what can occur during a PE journey in a group comparison format.