Variability in the electrocardiogram (ECG) can be due to extrinsic noise or can be caused by intrinsic factors, such as changes in the volume conductor or in the heart itself. Computer programs for the interpretation of the ECG base their diagnostic classification on one set of measurements that is derived from a representative PQRST complex or that is computed by taking the median from the measurements for each complex in the recording. However, these methods may fail to do justice to the intrinsic variability that may be present in the ECG. An alternative method is proposed: derive a set of measurements from each complex in the recording, classify each individual complex separately, and then combine the individual classifications into one final classification. This procedure has been evaluated on a validated database (n = 1,220) using an ECG computer program. Total accuracy against the clinical evidence increased from 69.8% for the interpretations of the averaged complexes to 71.2% for the combined interpretations of the individual complexes (p < 0.001). The effect of beat-to-beat variation on the measurements and classifications is demonstrated and the influence of extrinsic and intrinsic variability is assessed.