A computer implemented method to identify one or more areas of the heart muscle responsible for supporting or initiating abnormal heart rhythms using electrogram activation data recorded from a plurality of electrodes obtained from a corresponding series of sensing locations on the heart over a recording time period;, the electrogram activation data including data on a plurality of activations and further including activation signals of the heart muscle the method including the steps of: a) setting a pre-defined geodesic distance, b) dividing the recording time period into several analysis time periods, and pairing each sensing location with a plurality of other sensing locations from within the defined geodesic distance, thus forming a plurality of location pairings; c) for each of the analysis time periods, defining the relative timing of each activation signal for each location within each pairing, d) determining whether the relative timing of activation signals falls within plausible biological parameters, e) defining a leading signal of the pair for each activation within the respective analysis time period, the leading signal having properties that lead in the pair; and f) assigning a series of lead signal scores to each electrogram pairing acquired within each analysis time period based on the proportion of time within the respective analysis time period that each activation signal is leading within each pairing; g) repeating, at least once, the analysis steps b-f, each repetition being for the same sensing locations and for analysis time periods overlapping with analysis time periods of at least one other of the repetitions; h) combining each analysis time period for each signal location to provide a statistical measure of the proportion that each signal location tends to lead relative to other locations within the defined geodesic area; and i) relating lead signal scores from overlapping geodesic areas to provide relative combined lead signal scores; to provide an indication of the relative likelihood that each sensing location is generally preceding other areas and is therefore at or adjacent to a driver area of the abnormal heart rhythm. a) setting a pre-defined geodesic distance, b) dividing the recording time period into several analysis time periods, and pairing each sensing location with a plurality of other sensing locations from within the defined geodesic distance, thus forming a plurality of location pairings; c) for each of the analysis time periods, defining the relative timing of each activation signal for each location within each pairing, d) determining whether the relative timing of activation signals falls within plausible biological parameters, e) defining a leading signal of the pair for each activation within the respective analysis time period, the leading signal having properties that lead in the pair; and f) assigning a series of lead signal scores to each electrogram pairing acquired within each analysis time period based on the proportion of time within the respective analysis time period that each activation signal is leading within each pairing; g) repeating, at least once, the analysis steps b-f, each repetition being for the same sensing locations and for analysis time periods overlapping with analysis time periods of at least one other of the repetitions; h) combining each analysis time period for each signal location to provide a statistical measure of the proportion that each signal location tends to lead relative to other locations within the defined geodesic area; and i) relating lead signal scores from overlapping geodesic areas to provide relative combined lead signal scores; to provide an indication of the relative likelihood that each sensing location is generally preceding other areas and is therefore at or adjacent to a driver area of the abnormal heart rhythm.