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We present a statistical analysis of a sky survey of interstellar H 2 O and OH masers. These masers can be classified into three categories: isolated H 2 O masers, isolated OH masers, and simple OH/H 2 O maser associations. The total number of sources in each category is of the same order of magnitude, and as an evolutionary phase they can maintain ∼ 10 5 yr. An improved radiative pumping mechanism is proposed. This model avoids some of the deficiencies of previous radiative models, such as shortage of exciting photons. The statistical results obtained from the survey can be interpreted by the new mechanism together with the evolutionary model in which the gravitational force of the central stellar objects is responsible for the HII region.