Final answer:
The independent variable in Gabriel's experiment is the addition of robots to assembly lines, with the dependent variable being the productivity of employees on those lines.
Step-by-step explanation:
The independent variable in Gabriel's experiment, which investigates the effects on human performance when robots are added to assembly lines, is the presence or addition of robots to the assembly lines. This variable is being manipulated by the experimenter (Gabriel) to observe the effect it has on the dependent variable, which is the productivity of the participants working on the assembly lines.
Gabriel predicts that the productivity will increase with the integration of robots, thus defining the purpose of the experimental study. The independent variable is the cause, while the dependent variable is the effect or outcome that is being measured, which in this case is the productivity of factory workers. In Gabriel's study, he must ensure that any other potential factors that could affect productivity are controlled, so the effects measured can be confidently attributed to the introduction of robots.