Williams' AD is the accumulated sum of positive "accumulational" and negative "distributional" price movements. For example, if the current closing price is higher than the previous one, W/AD increases by the difference between the current closing price and the true minimum. If the current closing price is lower than the previous one, W/AD decreases by the difference between the current closing price and the true maximum.
The term "accumulation" denotes a market controlled by purchasers and the term "distribution" means that sellers control the market.
Divergences between the indicator and the price are a signals. Like most indicators, W/AD leads the tool price. In other words, when a divergence appears, the price changes its direction according to the indicator.
If the price reaches a new maximum, but the accumulation/distribution indicator cannot reach a new maximum, it means that the security is distributing itself. It is a signal for sell.
If the price reaches a new minimum, but the accumulation/distribution indicator cannot reach a new minimum, it means that the security is accumulating. It is a signal for buy.
To calculate the accumulation/distribution indicator, first you have to find a "True Range High" (TRH) and "True Range Low" (TRL):
Then you must find the current value of accumulation/distribution (CurA/D) by comparing today and yesterday's closing prices.
Williams' accumulation/distribution indicator is a growing sum of these values for each day: