Sampling
The Greater London Authority funding pupils’ attendance at the zoo offered a unique opportunity to study patterns of zoo-based educational impact without the potential selection bias of ability to pay that would normally apply. Moreover, the fact that there was a split in the population of visiting pupils between those whose visits were supplemented by an educational presentation tailored to the zoo context and those whose attendance was unguided offered the opportunity to assess whether such additional zoo education made any difference and whether pupils visiting without such supplementary education still learned anything of value.