5.1. EWMA chart results
The ARL curves obtained using the EWMA chart for a several values of λ are shown in Fig. 6a for a step-type shift, and in Fig. 6b for ramp-type underperformances. The values smoothing Parameter λ of the EWMA we used are 0.1, 0.3 and 0.5, for the purpose of the detecting a long-term phenomenon. A yearly 1% shift could possibly have been detected over an average of 234 days, using λ = 0.1. It came as no surprise that for step underperformance, the ARL values were smaller, because the underperformance reached its nominal value instantly. Detection was accomplished over an average of 80 days. The higher the λ value, the higher the ARL. As mentioned earlier, if λ = 1, the EWMA chart becomes a Shewhart chart, so the EWMA chart is clearly more efficient in cases of an underperformance detections smaller than 10%. The errorbars are the standard deviation of the detection time's distribution time over the number of runs required to reach convergence of the ARL.