A researcher conducted an experiment to test whether exposure to high levels of UV radiation (that seen when ozone depletion is at its peak) harms human skin cells. He exposed human skin cells growing in Petri dishes to this level of UV radiation and determined the percentage of cells in each dish that died. He found that 30% of the cells died after 3 days of exposure to high UV radiation exposure. Why would it be unreasonable to conclude that this level of UV radiation exposure is harmful to these cells?

It be unreasonable to conclude that this level of UV radiation exposure is harmful to these cells because, There is no control group and we don't know how many cells would have died after 3 days with normal UV exposure.

Explanation:

The researcher in the given example shows the effect of UV radiation and its effects to the cells. He does not tell about the group that did not receive the UV radiation and their effects. A control group is very important in a research because it can be compared with the result obtained by some experiment effects. It is a group in an experiment in which the treatment is not given.

The results can be compared with the group under treatment with the group that did not undergo any treatment. Since, there is no control group in this experiment, we can not determine how many cells that were exposed to normal UV radiation died after 3 days, it would be unreasonable for getting the conclusion of how harmful UV radiation exposure to these cells.