A data encoding method for encoding successive input data bits comprises the steps of: selecting one of two complementary sub-ranges of a set of code values according to whether a current input data bit is a one or a zero, the proportions of the two sub-ranges relative to the set of code values being defined by a context variable associated with that input data bit; assigning the current input data bit to a code value within the selected sub-range; modifying the set of code values in dependence upon the assigned code value and the size of the selected sub-range; detecting whether the set of code values is less than a predetermined minimum size and if so: successively increasing the size of the set of code values until it has at least the predetermined minimum size; and outputting an encoded data bit in response to each such size-increasing operation; modifying the context variable, for use in respect of a next input data bit, so as to increase the proportion of the set of code values in the sub-range which was selected for the current data bit by the greater of a predetermined minimum increment; and an increment derived in dependence upon a predetermined fractional amount of the current proportion.