Transplanting Lettuce Seedlings Help?

[chrisred83]

I'm a student at a University in the Caribbean and we've been given a PBL (problem based Learning) project on Transplanting Lettuce Seedlings.
This is it>> Problem Area- What is the best age for transplanting lettuce seedlings?
Situation- Farmer Michael purchased same variety lettuce seedlings, in styrofoam speedling trays, from 2 different sources. They were transplanted and after 6 weeks he found the heads were of different sizes.

i need to get info on: What could have caused this problem, for a named variety, and possible solutions to avoid a recurrence....
Thanks for any help!!