Question

Researchers conducted an experiment to identify the effects of three different hand sanitizers on the growth of bacteria. The researchers grew one type of bacteria in twenty-four identical covered petri dishes. Then, they opened eighteen petri dishes and applied the three hand sanitizers to six petri dishes each. They left the remaining six petri dishes closed as a control group. When the researchers measured the growth of bacteria two days later, the six dishes that had no hand sanitizer showed the least amount of bacterial growth. The researchers concluded that not using any hand sanitizer would lead to the least bacterial growth. Which statement best explains why the researchers’ conclusion was not justified?

Answer 1

The research had used 24 petri dishes which were identical from the beginning.

Unfortunately during application of the hand sanitizer to 18 of this dishes (6 for each type of sanitizer) but did not include opening of the other 6 petri dishes which should have provided this 24 dishes similar conditions as they were all exposed to the environment. On list the experiment would be more conclusive if the following conditions were applied

Sanitizer A -  6 petri opened and applied with sanitizer
Sanitizer B -  6 petri opened and applied with sanitizer
Sanitizer C -  6 petri opened and applied with sanitizer
No Sanitizer - 6 petri opened with no applied sanitizer