An experiment was designed to investigate the effect of caffeine on the heartbeat of water fleas. Two populations of water fleas were cultured. Both populations had water with the same mineral content, were supplied with identical amounts of bacteria as food, received the same amount of light, and had their temperature maintained at 20ºC. Every two hours, water fleas from both populations were selected and their heartbeats were monitored. The fleas of population one had caffeine administered five minutes before their heartbeat was checked. The fleas of population two were given nothing. what is the independent variable?
what id the dependent variable?
two factors that were constants for both groups.