A computer implemented method of determining that at least two members of a subscription group shared a journey from an origin to a destination over a period of time, the method comprising the steps of: obtaining location and time data defining a first plurality of journey positions (202...204) from the origin to the destination of a first mobile electronic device associated with a first member; forming an acceptable geographical-time zone (210..220); obtaining location and time data defining a second plurality of journey positions from the origin to the destination of a second mobile electronic device associated with a second member; determining whether each second mobile electronic device journey position falls within the acceptable geographical-time zone (210..220) providing an indication that the first and second mobile electronic devices shared a journey; the method being characterized in that it includes: segmenting the plurality of journey positions into a plurality of journey segments; forming an acceptable geographical-time zone (210..220) around each journey segment; determining whether each second mobile electronic device journey position falls within the acceptable geographical-time zone (210..220) around each journey segment; re-segmenting a journey segment into sub-segments where a journey position of the first mobile electronic device falls outside of the acceptable geographical-time zone (210..220) of the journey segment; and forming an acceptable geographical-time zone around each journey sub-segment, wherein a journey segment is re-segmented into sub-segments when the change of direction is greater than 40 degrees. obtaining location and time data defining a first plurality of journey positions (202...204) from the origin to the destination of a first mobile electronic device associated with a first member; forming an acceptable geographical-time zone (210..220); obtaining location and time data defining a second plurality of journey positions from the origin to the destination of a second mobile electronic device associated with a second member; determining whether each second mobile electronic device journey position falls within the acceptable geographical-time zone (210..220) providing an indication that the first and second mobile electronic devices shared a journey; the method being characterized in that it includes: segmenting the plurality of journey positions into a plurality of journey segments; forming an acceptable geographical-time zone (210..220) around each journey segment; determining whether each second mobile electronic device journey position falls within the acceptable geographical-time zone (210..220) around each journey segment; re-segmenting a journey segment into sub-segments where a journey position of the first mobile electronic device falls outside of the acceptable geographical-time zone (210..220) of the journey segment; and forming an acceptable geographical-time zone around each journey sub-segment, wherein a journey segment is re-segmented into sub-segments when the change of direction is greater than 40 degrees. segmenting the plurality of journey positions into a plurality of journey segments; forming an acceptable geographical-time zone (210..220) around each journey segment; determining whether each second mobile electronic device journey position falls within the acceptable geographical-time zone (210..220) around each journey segment; re-segmenting a journey segment into sub-segments where a journey position of the first mobile electronic device falls outside of the acceptable geographical-time zone (210..220) of the journey segment; and forming an acceptable geographical-time zone around each journey sub-segment, wherein a journey segment is re-segmented into sub-segments when the change of direction is greater than 40 degrees.