AD Percent = (Advances Less Declines) / Total Issues * 100
The Advance-Decline Percent fluctuates between 100% and -100%. 100% means that all stocks in the group in this case the S&P 500 advanced. The AD Percent chart is very choppy so to use a moving average to smooth it out makes sense. The chart below shows %SPXADP with the 21 period moving average. The -3% and 3% horizontal lines separate the bearish and bullish zones. Moving below -3% the bearish treshhold would be bearish for the S%P 500.