Answers may vary. One of the possible lurking variables can be as follows:

An observational study conducted, suggested that vitamin E reduces that the risk of heart disease and also experiments showed that vitamin E has no effect in reducing the risk.

It is mentioned that, an observational study concluded that ‘vitamin E reduces that the risk of heart disease’ this conclusion might be reached because of the effect of the lurking variables in the study.

One of the lurking variables that affect the results is that, individuals in the study who are health conscious, this indicates that these individuals might take vitamin E for some other reasons. Also, the researchers who conducted the study might only include the people who are more health conscious. Thus, the study would be bias towards the people who are health conscious. Another lurking variable can be that individuals who are wealthy can buy vitamins for their health.

Thus, the lurking variables that affected the results of the study are individuals who are health conscious and wealthy.

In randomized experiment all the individuals in the study are assigned randomly to the treatments. In this study some of the individuals are randomly administered to vitamin E (treatment) and some the individuals are randomly not administered to vitamin E. Because of the randomly assigned the treatments, the results obtained in the experimental study would be more appropriate than observational study.

Moreover in observational study only the individuals are just observed and conclusion is drawn and also has some affects of lurking variables, All these reasons might lead to the people who take vitamin E supplements have better health in observational studies but not in experiments.