In a visitor counting system a plurality of sensors (107, 108, 109, 110) count the number of visitors passing by. A data-recording device (105, 106) connected to the sensors store records comprising the number of visitors counted within a predetermined time period. A remote visitor data processing unit (120) further comprises a record validation block for checking validity of the records, an interpolation block for creating new records to substitute the incorrect records, and a faulty-sensor detection block for concluding, based on the records, whether a sensor is faulty.