Williams Accumulation/Distribution was created by Larry Williams. This indicator measures the pressure to Sell/Buy. That is also the reason why it's called Accumulation (Bulls control the market) and Distribution (the market is controlled by Bears).
Williams Accumulation/Distribution formula:
If Close n > Close n-1
AD = AD n-1 + [Close – MIN (LOW, Close n-1)]
If Close n < Close n-1
AD = AD n-1 + [Close – MAX (HIGH, Close n-)]
If Close = Close n-1
AD = AD n-1
How to use the indicator:
As you can see from the indicator's construction, traders are mostly looking for the positive and negative divergences with the price chart. If the price rises, but indicator value falls, the preasure to Sell rises. We can expect the trend reversal from Uptrend to a Downtrend. If the price falls but indicator value rises then we can expect the Downtrend being changed to an Uptrend.
Copyright © Picture made by Incredible Charts
As you can see in the picture above, there is a Williams Accumulation/Distribution plotted first and then Williams Accumulate/Distribute second.
Note: In the first version of the indicator the daily WA/D was multiplied by Volume of the asset. After the multiplication it was added to the yesterday's WA/D. This calculation was adjusted by Steven Achelis in such a way that it doesn't take the Volume into consideration any more. Nowadays the Achelis modification is mostly used so the Volume is not implied.
According to the information source you can find two names of the indicator - Williams Accumulation/Distribution as for the first William's indicator and Williams Accumulate/Distribute as for the Achelis adaptation. But it is quite rare to meet both the indicators as the Achelis version is used almost everywhere, even under the name Williams Accumulation/Distribution.