An experimenter wants to know which type of light bulb promotes the most plant growth. He plants 50 pea plants in a room under GE brand light bulbs and another 50 pea plants under Phillips brand light bulbs. Water, temperature, soil type, and other conditions are kept the same between both groups of plants. He allows both groups of plants to grow for 30 days and records the growth of each plant twice daily. The rate at which the plants grow is the