Dr Mokoena believes that people who are under stress will develop more colds than people who are not under stress. When she randomly selects 10 participants and exposes them to high levels of stress, she finds that 9 of the participants develop colds. Based on these results she concludes that stress causes an increase in colds. Dr Mokoena's reasoning may be flawed because in this study

Question

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Dr Mokoena did not include a control group, which means she did not have a comparison group to see if the rate of colds was higher in the group exposed to stress compared to a group not exposed to stress. Without a control group, it is impossible to determine if the observed effect is due to the independent variable (stress) or to other factors.

Dr Mokoena Believes That People Who Are Under Stress Will