Dr. Stevens observes that squirrels in the wild eat nuts, fruit, and insects. She wonders which of the food sources makes young squirrels grow the fastest. She separates thirty young squirrels into two equal groups. She feeds one of the groups nuts and she feeds the other group insects. The group of squirrels that eats the nuts grows faster than the group of squirrels that eats the insects.

What question could Dr. Stevens ask next if she wants to continue studying about squirrel growth rate?