The formula:
FI( t ) = V( t ) * ( C( t ) - C( t - 1 ) )
FIS( ) = Exponential Smoothing of FI( ) to NP.
where:
FI( t ) = pure force index of the current day
FIS( ) = force index with Exponential Smoothing
V( t ) = today's volumes
C( t ) = today's closing
C( t - 1 ) = yesterday's closing
NP = number of periods (single parameter of the indicator)
Two warnings when using this indicator:
1) It's preferable to draw the Force Index as a histogram;
2) Moving Averages can be added to the Force Index to help generate signals; it is preferable to choose an exponential moving average at 2 days, as well as on the one at 13 days: the first one provides excellent levels of input and output, the second one allows you to highlight the long-term trend changes.
Prices moving above the zero-line are seen as positive or could begin a bullish move, while prices moving below the zero-line are seen as negative or could be beginning a bearish move.