The combination of photochemotherapy with oral methoxisalen (psoralen) and ultraviolet A radiation called PUVA treatment is an effective treatment for psoriasis. However, PUVA is mutagenic, increases the risk of squamous-cell skin cancer, and can cause irregular, pigmented skin lesions. Stern et al. performed a study to assess the incidence of melanoma among patients treated with PUVA. The study identified 1380 patients with psoriasis who were first treated with pUVA in 1975 to 1976. The patients were subdivided according to the total number of treatments received (<250 or >/ (greater than,equal to) from 1975 to 1996) Within each group the observed number of melanomas was determined from 1975 to 1996 and compared with the expected number of melanomas as determined by published U.S. age- and sex-specific melanoma incidence rates. The results were in Table 7.6
7.6 Relationship of PUVA treatment to incidence of melanoma.
Observed Expected
< 250 treatments 5 3.7
>/ 250 treatments 6 1.1
Suppose we want to compare the observed and expected number of events among the group with <250 treatments. Perform an appropriate significance test, and report a two tailed p value.