Absolute, or perfect, pitch—the ability to identify the pitch of an isolated musical note—is rare in the general population, but relatively common among trained musicians.  A survey of fifteen-year-old music students showed that absolute pitch was more common among those who had been studying music since a very early age than among those who started studying a few years later.  Thus very early musical training aids in the development of absolute pitch.

 

Which of the following, if true, most seriously weakens the argument?