What it means
Welles Wilder introduced the Swing Index and its cumulative sibling in his 1978 book New Concepts in Technical Trading Systems, the same work that produced the RSI and the Average True Range. The Accumulative Swing Index, or ASI, is the long-running total of the daily Swing Index readings.
The Swing Index compresses each day's open, high, low and close into one number. It compares today's prices with yesterday's and weights gaps and the day's range, so it measures the real strength of a move.
A strong positive reading means buyers controlled the session. The ASI simply adds those daily scores together.
The result is a single line that behaves like a gap-adjusted price line, designed to show the market's true push without the noise of individual sessions. Rising totals suggest an uptrend and falling totals a downtrend.
Traders read the ASI against price. When price makes a new high but the ASI does not, the divergence warns that the move lacks force.
When both break a prior swing together, the signal is confirmed. Trendline breaks on the ASI get special attention.
Because the index digests gaps and limit moves, a break on the ASI often appears before the same break on the price chart, giving an early hint that a trend is turning. The indicator is a creature of futures markets, where gaps and limit moves are common.
Its weakness is complexity: the formula is fiddly, parameter choices change the picture, and like all trend tools it whipsaws in sideways markets.
In practice
Real-world examples.
Example
A stock closes at a record high, but its ASI prints a lower high. A technician reads the bearish divergence as a warning and trims the position into strength before any pullback appears on the price chart.
Example
The ASI of a crude oil contract breaks above a month-old downtrend line while price still sits below its own resistance. The trader treats the early break as a buy signal and places a stop under the recent low.
Example
In a choppy sideways currency market, the ASI flips direction every week. The trader stands down, recognising the indicator's known weakness in range-bound conditions.
Formula
Calculation
ASI = previous ASI + today's Swing Index. In simplified form, with C, O, H and L for today and CY and OY for yesterday's close and open, N = (C - CY) + 0.5 x (C - O) + 0.25 x (CY - OY), and SI = 50 x (N / R) x (K / T). Here R is a range measure built from the largest of H - CY, CY - L and H - L, K is the larger of H - CY and CY - L, and T is the limit move.
Take CY = 100, OY = 100, O = 100, C = 104, H = 106, L = 98 and T = 12. Then N = 4 + 2 + 0 = 6, the largest range is H - L = 8, so R = 8 + 0.25 x 0 = 8, and K = 6. SI = 50 x (6 / 8) x (6 / 12) = 50 x 0.75 x 0.5 = 18.75. If the prior ASI was 120, today's ASI = 120 + 18.75 = 138.75. Wilder's book gives the full rules.Case study
Seen in the real world.
In this fictional case, an invented grain trader named Dara watches soybeans push to a new contract high while the ASI stalls below its own prior peak. Suspecting exhaustion, she tightens her stops instead of adding to the position.
Two sessions later the price gaps down, and the ASI trendline had broken first. The early warning does not make the indicator infallible, but it saved her long position from the worst of the drop.
Afterwards she reviews her trade log and finds that the ASI divergence has helped in some trades and misled her in others. She decides to use it only as a warning to tighten risk, never as a standalone entry signal.
Watch out
Common mistakes.
- Trading the ASI alone in ranging markets, where it whipsaws; combine it with a trend filter or use it only when a clear trend exists.
- Ignoring divergences, which are the indicator's main value; treating every tick of the line as a signal discards its best feature.
- Misimplementing the formula; the range scaling and limit-move factor are easy to code wrong, so test any version against Wilder's worked examples.
Questions
People also ask.
What is the Accumulative Swing Index?
It is Wilder's running total of his daily Swing Index readings. It condenses open, high, low and close into a single line used to confirm trends and spot divergences.
How do traders use the ASI?
They compare it with price, since a new price high with a lower ASI high warns of a weakening trend. Trendline breaks on the ASI sometimes lead price breakouts, and it is most useful in trending, gap-prone markets such as futures.
What is the difference between the Swing Index and the ASI?
The Swing Index scores a single session, adjusting for gaps and range. The ASI adds those scores over time into one long-run line.
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