On the first day of testing, have your test subjects trim their nails to a short length. The nails should be bare.
In weekly intervals, measure how much the nail has grown from the last week for your test subjects. Do this for one month.
At the one month mark, have your test subject trim their nails again to the same length it was on step 1.
Apply some nail polish on all the nails.
In weekly intervals measure how much the nail has grown from the last week for your test subjects. Do this for one month.
If the nail polish chips, then reapply. The nail polish should cover the nails from cuticle to tip.
Repeat the above steps for two more months.
Compare your data and evaluate your results.