Skin cancer prevention

Skin cancer is the most common cause of cancer in the United States. Many forms of sunscreen help prevent against UV-B radiation which has been linked to basal and squamous cell carcinoma. Most sunscreens, however, notably lack any protection from UV-A radiation which is known to cause a more severe form of skin cancer known as malignant melanoma. Zinc oxide based compounds have shown efficacy at repelling UV-A radiation, thus decreasing the risk of malignant melanoma.

One dermatologist is in the process of developing a new form of sunscreen that incorporates zinc oxide to protect against UV-A and UV-B radiation. She has designed an experiment where half of the subjects apply her novel sunscreen while the other half apply standard sunscreen every day for four and a half years. Following the study, she finds that $2\mathrm{\%}$‍ of the subjects applying her novel sunscreen developed malignant melanoma while the occurrence was $8\mathrm{\%}$‍ in those applying the standard sunscreen.