The Relative Strength Line Nobody Taught You to Read

The line that isn't about price at all

Every stock chart shows you the same thing: price, over time. A line going up, a line going down, candles, volume underneath. It's the single most common piece of information in investing, and it's also — this is the part that took me a while to really sit with — not actually the thing that predicts whether a stock is about to move.

There's a second line, far less commonly plotted, that measures something different: not whether a stock is going up, but whether it's beating the market while it does. It's called Mansfield Relative Strength, and once you understand what it's actually built from, you start noticing how much the ordinary price chart leaves out.

Where it actually comes from

The name has a real history, and it's older than most people assume. R.W. Mansfield Co., based in Jersey City, New Jersey, published something called the Mansfield Stock Chart Service — a printed, loose-leaf volume of stock charts that circulated among traders going back to the early-to-mid 1900s, long before anyone had a screen to look at. Those charts carried a relative strength line alongside price, plotted by hand or by early printing methods, for anyone willing to do the work of comparing a stock's performance to the broader market.

The formula didn't get formalized and popularized the way we use it today until 1988, when Stan Weinstein wrote it into Secrets for Profiting in Bull and Bear Markets — a book I've actually reviewed here before as part of building out a trading library. Weinstein took the Mansfield chart service's relative strength concept, gave it a precise mathematical definition, and built an entire stage-based framework for reading stocks around it. That's the version that stuck, and it's the one still called "Mansfield RS" today — a name borrowed from a chart service most current traders have never seen a single page of.

The actual formula

Strip away the jargon and it's genuinely simple. First, you need what's sometimes called Dorsey Relative Strength (DRS) — just the stock's price divided by the index's price:

DRS = Stock Price ÷ Index Price

That ratio alone tells you very little on its own — it's just a number that moves around with both prices. The Mansfield step is comparing today's DRS to its own recent average:

Mansfield RS = ((DRS today ÷ SMA of DRS over n periods) − 1) × 100

Weinstein's original defaults: n = 52 on weekly charts, n = 200 on daily charts. Some modern implementations use 252 (roughly a trading year) instead of 200 for daily — close enough to the spirit of "about a year," and honestly not worth arguing over.

What that formula is actually doing: it's asking whether the stock-vs-index ratio right now is above or below where that same ratio has been sitting, on average, over the past year. Not "is the stock up." Not "is the stock beating the index today." Specifically: is the stock's relationship to the index currently stronger than its own recent normal.

How to actually read it

• Positive value → the stock is currently outperforming the index relative to its own recent trend. It's stronger than its own normal, not just green today.
• Negative value → the opposite — underperforming relative to its own recent baseline, even if the stock's price is technically still going up.
• Rising line → the outperformance is building, whether the reading is above or below zero.
• Falling line → the relative advantage is eroding, again regardless of the current sign.
• The zero-line cross → this is the signal Weinstein actually cared about most. A stock crossing from negative to positive RS is quietly telling you something the price chart usually hasn't confirmed yet.

That last point is the entire reason this indicator is worth learning. A stock can sit dead flat in price — genuinely boring, nothing happening — while its Mansfield RS quietly crosses from negative to positive, because the index is falling faster than the stock is. Price alone would never tell you that. The ratio does.

The mistake almost everyone makes with it

The most common way to misread Mansfield RS is treating a positive number as "good" and a negative number as "bad," full stop — as if it were just another momentum oscillator. It isn't. A stock can have negative Mansfield RS and still be in a strong uptrend; it just means the index is up even more. And a stock can have positive RS while its own price is falling, if the broader market is falling faster.

The number isn't graded against zero in isolation — it's graded against its own recent history, and it's always relative to whatever benchmark you chose. Change the benchmark (a sector index instead of the broad market, say) and the same stock's Mansfield RS reading can flip. That's not a bug — it's the entire value of the indicator: it forces you to ask "relative to what," a question a plain price chart never makes you ask at all.

Daily, weekly, or monthly — which one matters

Weinstein built this for weekly charts, and that's still the right home base for it — stage analysis is fundamentally a multi-month framework, and a 52-week lookback matches that horizon. But there's real value in checking more than one timeframe together:

• Weekly RS tells you the structural picture — is this stock's relationship to the market improving or decaying over the medium term.
• Daily RS catches the earlier signal — because it reacts faster, a stock can flip positive on daily RS weeks before the weekly reading confirms it.
• Monthly RS is the slowest and most conviction-heavy read — useful for separating genuine multi-quarter leadership from a stock having a good few weeks.

None of the three is "correct" on its own. The strongest setups are the ones where daily RS turns positive first, weekly RS confirms not long after, and price is still sitting flat while all of this is happening underneath it — which is exactly the "quiet accumulation" window that gets talked about constantly in trading circles without anyone explaining what number is actually doing the talking.

Why this is worth the ten minutes it takes to understand

Nobody teaches this in school, and it's not exactly hard once someone walks you through it — which is precisely why it's frustrating that so few people ever get the walkthrough. A number that's been sitting quietly on printed charts since before most of us were born, formalized in a book from 1988, still isn't part of how most retail investors are taught to look at a stock. The chart everyone stares at is price. The chart that actually tells you what's happening underneath price is this one.

Related: the full review of Secrets for Profiting in Bull and Bear Markets covers Weinstein's stage-analysis framework this indicator was built for, and Catch Wealth Before It Runs shows what this same number looks like applied to 35 real stocks across every sector.