Average number of flights to solo for ab-initio students?

My club uses the volunteer instructor model for student instruction,
and I am trying to determine how effective this model is versus
commercial soaring center instruction. Based on my review of our
records (such as they are) it appears it takes our students an average
of 33 flights from 'first ride' to solo.

Anyone else out there have similar objective data from their
operations?

TIA

Frank (TA)