13. Battery life Does reducing screen brightness increase battery life in laptop computers? To find out, research
ers obtained 30 new laptops of the same brand. They
chose 15 of the computers at random and adjusted
their screens to the brightest setting. The other 15 lap.
top screens were left at the default setting--moder
ate brightness. Researchers then measured how long
each machine's battery lasted. Is this an observational
study or an experiment? Explain your reasoning.