Asthma is an extremely common illness with a high morbidity and 
significant mortality. The UK has the highest number of asthma 
sufferers in Europe, with 15% of the population affected to some
degree.  It is generally accepted that part of the effective treatment of asthmatic patients is the monitoring of their condition.
Self-measurement of lung function by patients enables clinicians to
assess the severity of the illness and allows the treatment to be
tailored to the patient's needs. Measurements most commonly used in
General Practice are peak expiratory flow readings using a Wright's peak flow meter. Patients are asked to record measurements twice daily and enter them on a peak flow graph in a patient diary. This system of recording depends not only on the patients remembering the correct figures but also on them entering the data accurately on the graph afterwards. There is no way that the clinician treating the asthmatic patient can be entirely sure that the figure and the corresponding entry on the graph are accurate representations of peak flow at the time. The results are always viewed retrospectively by the clinician who looks for trends since the last visit to the Asthma Clinic, hence the figures provide little information with regard to the patient's condition at that particular time and they have limited predictive value.