In an investigation designed to determine the effect of the amount of water on plant growth, two groups of equal-sized bean plants of the same species were grown under identical conditions, except for the amount of water they were given. One group was watered with 200 milliliters of water once a day, while the other group was watered with 400 milliliters of water once a day. After several days, the height of plants were measured. It was determined that the plants watered with 400 milliliters of water once a day showed more growth. The test variable (independent variable) in this investigation is the