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7.63,,The combination of photochemotheraphy with oral methoxsalen (psoralen) and ultraviolet a radiation (called PUAVA 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 or 1976. Patients were subdivided according to the total number of treatments received (<250 or > 250 from 1975 to 1976). 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 as in table 7.6,, observed Expected,< 250 treatments 5 3.7,> 250 treatment 6 1.1,,7.63) Suppose we want to compare the observed and expected number of events among the group with < 250 treatment. Perform an appropriate significance test, and report a two- tailed p-value.,