A company that sold tulip bulbs asked a select group of customers to test a new variety of tulips in different climates. Sixty customers received bulbs and were given instructions on how to plant and care for the tulips. The customers were grouped according by the type of climate where the customer lived. One climate type had dismal results - very few of the bulbs lived to produce tulip flowers in the spring. Discuss factors that may have affected the results of this study