You really have asked a wonderful question. First of all, I must tell you that moving average is a magic indicator. Now lets come to the point. I have been using 3 moving average on my chart with 50, 100, 200. This is how you can easily understand that even if 50 fails, 100 keeps the stand. That goes with 200 too. On a lower time frame, we often find that 50 fails whereas this is not the case. You will see lower time frame even obey 100 at 80% cases and 200 at 99% cases.