Oscillators: Reading Force Rather Than Price

The whole oscillator in one line

RSI is 100 × average gain ÷ (average gain + average loss) over the last n periods, most often fourteen. Written that way, the mystery disappears: it is the ratio of two average moves, rescaled so that a balance sits at 50 and a total absence of losses sits at 100. An average gain of $0.89 against an average loss of $0.36 gives a ratio of 2.47 and a reading of 71.2, and those three numbers contain everything the indicator knows. The stochastic is the same idea measured differently — where the close sits inside the high-low range of the last n periods — so 80 means the close is near the top of that range and 20 means it is near the bottom. That construction is why the thresholds are descriptions rather than instructions. A reading of 70 is a ratio of 2.33, and reaching it requires the average gain over the window to be more than twice the average loss; nothing about that arithmetic forces a decline. To bring the reading to a neutral 50 the average gain would have to fall by about 60%, from $0.89 to $0.36 — a collapse of the up-momentum, not a pause. To push it to 30 with the gain unchanged, the average loss would have to triple to $2.08. Put those figures next to the claim that RSI 70 means “sell” and the claim falls apart: the indicator would be near 70 for the entire duration of a strong trend, which is precisely what it does, and traders who fade it are fading the strongest part of a move. So what is the indicator for? It is for comparing force over time within one instrument. That is what makes divergence the useful reading: when price makes a higher high while RSI makes a lower high, the second push is being made with less average gain relative to average loss, which is a measurable change in the character of the move. The same applies to the range version: a close at the top of its fourteen-period range after a long extension is a different proposition from the same close early in the move, and only the second is a strength signal. And it is for timing within a structure that already exists: an oscillator is a reason to prefer one entry over another inside a setup the structure and the regime already qualified, never a reason to have a view. One reading, three arithmetic facts — Average gain $0.89, average loss $0.36: Ratio 2.47 → RSI 71.2 · What RSI 70 is, in the same units: A ratio of 2.33 — the same statement, rescaled ← · To reach neutral 50: The average gain must fall 59.6% to $0.36 — a collapse, not a pause · To reach 30: The average loss must triple to $2.08 with the gain unchanged Shorten the window and the reading moves faster and lies more; lengthen it and the reading is more stable and later. As with moving averages, the lookback is the choice that matters, and the flavour is the decoration.

The two ways oscillators cost money

The first is fading a trend. In a sustained advance the reading sits above 70 for weeks, so a rule that sells every reading above 70 is short the best part of the move — repeatedly, and with the cost of each attempt. The error is a category mistake: the reading is a ratio, and a persistent ratio is exactly what a trend looks like from inside. The regime check that governs moving averages governs this too, and the version of it that is specific to oscillators is to treat the thresholds as *conditional*: in a trending market a high reading is a continuation reading, and in a range it is a warning that price is at the top of the range and the risk-to-reward of a long is poor. The second is trusting a divergence for timing. Divergences routinely appear several times before a trend ends, and each appearance looks like the one that finally worked. A divergence at the third test of a high in a market that has already doubled has different odds from a divergence in the first extension of a fresh trend, and the honest use is to let the divergence change the *plan* rather than open a position: reduce the size, move the stop to a tighter structural level, refuse to add, and require the break of structure that would independently have told you the trend was over. The divergence is then an input to risk, which is what it measures, and the position is still closed by structure, which is what actually defines the end of a trend. • In a trend, a high reading is a continuation reading; in a range it is a warning about the top of the range. • A divergence measures weakening force, not a reversal date, and often prints several times before a turn. • Use a divergence to change the plan — size, stop, no adds — rather than to open a counter-trend position. • The exit still comes from structure: the break that independently invalidates the trend. • Watch the reading against its own history on one instrument; the absolute level means little across instruments. The subtlest version of the mistake is comparing readings across instruments — “this one is only at 62 while that one is at 78” — as if the scale were absolute. RSI is a ratio measured over a fixed window on that instrument’s own price behaviour, so a stock with a large average move and one with a small move can show the same reading and be in entirely different conditions. The comparison that means something is a reading against the same reading on the same instrument over time, and against the structure it is sitting in.

The period is a free parameter

Every oscillator has a lookback — 14 is conventional for RSI, but it is convention, not science. Change 14 to 9 and the reading swings wider and signals more often; change it to 21 and the same setup produces a quieter line. The threshold that defines “overbought” moves with it, because a 9-period RSI reaches 80 far more readily than a 21-period one reaches 70. None of this is wrong; it is a choice, and a choice that is rarely recorded. The danger is degrees of freedom. A chart can be made to show a clean divergence sell signal by picking the lookback that makes it clean, and the same chart can be made to show the opposite signal with a different one. Searching parameters until the signal looks good, then describing the result as if it were a property of the market, is overfitting wearing an indicator’s clothes. The fix is the same as for patterns: fix the parameter before you look at the outcome, test it on data you did not use to choose it, and demand that it beat a benchmark that uses no indicator at all. There is a defensive use of the parameter that survives this critique. Because the period encodes how much smoothing you accept, you can choose it to match the holding horizon rather than to match a historical fit — a shorter lookback for a days-long trade, a longer one for a swing — and then leave it alone. An indicator with a stated, unoptimised period is a weaker claim, and a more honest one. If you have to try three settings to find the signal, you have not found a signal; you have found a setting.

Three indicators, one measurement

Put RSI, stochastics and MACD on the same chart and they will usually agree, and the agreement feels like confirmation. It is arithmetic. Every oscillator in this lesson is a function of the same handful of inputs — the closes over a recent window, and for the range versions the highs and lows as well — so RSI is a ratio of average gains to average losses, the stochastic is the close’s position inside a recent range, and MACD is the distance between two averages of the close. They are not three measurements of a market; they are three monotone transformations of one series, and two transforms of the same numbers cannot disagree about those numbers. “RSI is overbought, the stochastic is overbought and MACD is rolling over” is one observation counted three times, and adding a moving-average crossover, Williams %R or the commodity channel index makes it one observation counted five times. The cost is the same one the risk lessons attach to a portfolio: correlation is not diversification. Two oscillators built from overlapping windows of the same closes move together closely enough that the second adds a little information and the third adds approximately none, but each one adds confidence, and confidence that does not come from evidence is precisely what inflates position size at the wrong moment. A trader who feels three signals agree is likely to take a larger position than one who sees a single reading, and the incremental evidence for the larger position is close to zero. The honest test is the one worth applying to any claim of confirmation: could these indicators have disagreed here? If the answer is no — as it is for three functions of the same closes on the same window — then their agreement is not evidence about the market, it is evidence that the arithmetic was done correctly. What genuinely adds information is a different input rather than a different formula. Volume-based measures read participation, breadth measures read how many instruments are participating, implied volatility reads what options are pricing, and a longer timeframe reads the same price series over a longer window — which is not new data but is a different resolution, and therefore a real check in the sense that a signal which survives a longer frame is not an artefact of the shorter one. A ratio, such as a sector against the index, adds information because it is a comparison of two series rather than a transform of one. That is the practical rule for building an entry checklist: one oscillator on the frame you trade, and then anything that measures something else. Before treating a cluster of indicators as a confluence, count the distinct inputs behind them, exactly as the portfolio lessons count drivers rather than positions. • RSI, stochastics, MACD and the range oscillators are transforms of the same closes and highs and lows. • Three agreeing oscillators is one measurement counted three times, not three confirmations. • Agreement between indicators that cannot disagree is arithmetic, not evidence. • Add a different input — volume, breadth, implied volatility — or a different horizon, not another formula. • Count the distinct inputs behind a confluence the way you would count the drivers behind a portfolio. This does not make oscillators useless; it makes them one thing. The reading that carries information is divergence, and divergence on two different oscillators is largely the same statement about the same series. Choose one, learn its behaviour on your instrument, and spend the second indicator’s slot on something that can actually contradict it.

Sources

• RSI, stochastics and the construction of bounded oscillatorsMurphy, “Technical Analysis of the Financial Markets”
• The stochastic oscillator and range positioningLane, original stochastic oscillator publications
• Divergence, momentum and their measured reliabilityBulkowski, “Encyclopedia of Chart Patterns” (failure-rate data)