The analysis of the mean absolute error for rank order filters is discussed. These filters are a subset of a larger class of nonlinear filters, called stack filters, which obey two basic properties: a) a weak superposition property, the so-called threshold decomposition and b) an ordering property called the stacking property. This paper ranges among others representing a new approach to 2-dimension filtering especially suited for removing some disturbances ocurring in image transmission. A rich literature introduces the reader to this new trend in digital filtering.