Final answer:
The study is biased as it only tracked bicyclists with helmets and did not include a control group without helmets, which is crucial for a valid comparison.
Step-by-step explanation:
Based solely on the information given, there is reason to question the results of the hypothetical study. In a study designed to determine whether bicyclists who wear helmets have fewer accidents, researchers tracked 500 riders with helmets for one month.
The reason for questioning is that the study's sample is biased.
For a reliable study on whether bicyclists who wear helmets have fewer accidents, it is necessary to track both riders with helmets and those without helmets to establish a comparison group. Without including a control group of riders not wearing helmets, the study cannot properly assess the effect of helmets on the frequency of accidents.