A method of rank order filtering and a rank order filter apparatus is defined by an established rank order and accepts into a buffer, data points to be filtered, each data point having a data value and an associated time stamp. Data points are accepted until the buffer contains data points representing a minimum predefined time span. The filter calculating an amount of time the data values in the buffer are above an approximate filter value and adjusts the approximate filter value based upon the relationship of the amount of time to the rank order. The steps iterate to approach a value defined by the rank order and the approximate filtered value is output.