Theo was doing research to support the claim that exercise increases heart and lung health. He had 50 people do a rigorous exercise program. Each week, for six months, he tested their heart and respiratory rate. He determined that exercise does increase heart and lung health.

Theo tested everyone in the exercise program but did not compare their results with a group that did not exercise. Without a control group, he cannot tell whether changes in heart and respiratory health were caused by the exercise program or by other factors. So the study is invalid for supporting his claim because it lacked a control group.

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