What is relative strength in investing?

Relative strength measures how a security has performed compared with a benchmark or a peer group, rather than in absolute terms. It is usually computed as the ratio of the security's price to the benchmark's level over time, so a rising line means the security is outperforming and a falling line means it is lagging, regardless of whether either is going up.

What is the difference between relative strength and RSI?

They are unrelated despite the similar names. Relative strength compares one security against another security or an index. The Relative Strength Index, or RSI, is a single-security oscillator built from the size of recent up moves versus down moves, and it involves no benchmark at all.

How is relative strength different from absolute momentum?

Absolute momentum asks whether a security's own return over a window was positive. Relative strength asks whether it beat a benchmark over that window. A security can have strong relative strength while losing money, if the benchmark fell further, and it can have positive absolute momentum while lagging badly.

Relative Strength Explained: Measuring Performance Against a Benchmark

Relative strength measures how a security has performed against a benchmark rather than against zero. In its simplest form it is a ratio: the security’s price divided by the benchmark’s level, plotted over time. When that line rises, the security is outperforming the benchmark. When it falls, it is lagging. Whether either is actually going up is a separate question, and that separation is the whole point of the measure.

It is worth clearing up a naming collision immediately. Relative strength is not the Relative Strength Index. RSI is a single-security oscillator built from the size of recent gains versus recent losses and involves no benchmark whatsoever. The two share a name and nothing else.

How the ratio is computed

The base calculation is deliberately simple.

Relative strength line = security price / benchmark level

Both series must be measured on the same dates and on a comparable basis. In practice that means three things.

Adjust for corporate actions. The security’s price series has to be adjusted for splits, bonuses and similar events, or the ratio will show a cliff on the ex-date that has nothing to do with performance. See corporate actions and adjusted prices.

Match the total-return basis. If the benchmark you are using is a total return index, which includes dividends, and the security series excludes them, the ratio carries a small persistent drift that is an artefact rather than a signal. Comparing price index to price return, or total return to total return, keeps the ratio honest.

Rebase for readability. The absolute level of the ratio carries no information, because it depends on the units of both series. Analysts normally rebase the line to 100 at a chosen start date so the shape is readable and comparable across securities.

Read the line by its slope, not its level. A ratio of 0.8 tells you nothing on its own. A ratio that has risen steadily for six months tells you the security has been beating the benchmark over that period.

The variants you will meet

The plain ratio has several close relatives, and the differences matter when you are reading someone else’s chart or screen.

Relative return over a window. Rather than a line, a single number: the security’s return over a period minus the benchmark’s return over the same period. Simple, and the version most often used in ranking.

Ratio of cumulative growth. Both series indexed to 100 at a common start date, then divided. Identical in shape to the plain ratio, easier to read alongside the underlying series.

Normalised relative strength. The relative return divided by the volatility of the relative return over the same window, so that a steady lead ranks above a jumpy one that arrived at the same place. This is the same risk adjustment used in most published momentum rankings, described in how momentum is measured.

RS-Ratio and RS-Momentum. The two axes of a Relative Rotation Graph. RS-Ratio is a normalised measure of the relative strength level, and RS-Momentum is a normalised measure of the rate of change of that relative strength. Plotting them against each other produces the four-quadrant rotation picture explained in relative rotation graphs.

Cross-sectional rank. Instead of comparing to an index, rank every security in a universe by its own return over the window and express the result as a percentile. This is relative strength against a peer group rather than against a benchmark, and it is what most screening tools mean by the term.

Relative strength versus absolute momentum

These are different questions and confusing them is the most common error in this area.

Absolute momentum asks: was this security’s own return over the window positive? It is a question about direction. It has no benchmark and no peers in it.

Relative strength asks: did this security beat the benchmark or its peers over the window? It is a question about ranking. It says nothing about direction.

The four combinations are all real and all mean different things. A security can be up and beating the benchmark, which is the uncontroversial case. It can be up but lagging, which happens constantly in a strong market and is the ordinary experience of most holdings. It can be down but beating a benchmark that fell further, which is what defensive positioning looks like in a decline. And it can be down and lagging.

That third case is the one that catches people out. A screen ranking purely on relative strength during a broad decline will hand you a list of securities that have all lost money. Nothing in the measure prevents that, because nothing in the measure is looking at direction. Processes that care about both usually apply an absolute filter on top of the relative rank, for example requiring positive own-return over the window as well as a top-decile relative score.

What relative strength is used for

Sector and thematic rotation work. Plotting each sector index against the broad market shows which parts of the market have been leading and which have been lagging, which is the raw material of sector rotation analysis.

Ranking within a universe. Cross-sectional relative strength is the standard input to momentum screens, because it removes the common market move and leaves what is specific to each security.

Peer comparison in fundamental research. Comparing a company’s price line to a sector index over a period is a quick way to ask whether the market has been treating this business differently from its peers, which is a prompt for further research rather than an answer.

Fund and manager assessment. A fund’s relative strength against its stated benchmark is the visual form of the same information contained in active return and tracking error.

Choosing the benchmark, which decides everything

The benchmark is not a detail. It is the measurement.

The same security compared to a broad market index, to its sector index, and to a narrow peer basket can look strong, average and weak at once, and all three readings are arithmetically correct. Before drawing any conclusion from a relative strength line, be explicit about what it is relative to and why that is the right comparison for the question you are asking. If you would not accept the benchmark as a fair yardstick for judging performance, it is not a fair yardstick for judging relative strength either.

Consistency also matters. Changing the benchmark partway through a study, or comparing two securities against different benchmarks and then ranking them together, produces a number that means nothing.

What relative strength does not tell you

It says nothing about direction. Strong relative strength in a falling market is compatible with losing money. This is the single most important caveat.

It says nothing about valuation or business quality. The ratio is a price comparison. Earnings, balance sheet, governance and the durability of the business are all outside it.

It says nothing about why. The line moving up tells you the market treated this security better than the benchmark. It does not tell you whether that was earnings news, sector flows, an index inclusion, a one-off event, or a change in liquidity.

It is entirely window dependent. A security can show strong relative strength on a six month window and weak relative strength on a twelve month window at the same instant. Neither is wrong. Both are answers to different questions, and quoting one without stating the window is close to meaningless.

It is not predictive on its own. Relative strength is a description of the past. The claim that it tends to persist is a separate, contested, statistical claim with wide uncertainty and long stretches where it does not hold, as discussed under factor cyclicality and drawdowns.

It can be an artefact of construction. Mismatched dividend treatment, unadjusted corporate actions, currency differences or a benchmark that was reconstituted during the period can all produce ratio movements that have no economic meaning.

A relative strength line answers exactly one question: over this window, against this benchmark, who moved more. Every other interpretation you place on it is yours, not the data’s.

Portfolio and Backtest Metrics, Explained: the hub guide to portfolio and strategy metrics.
How to Identify Momentum Stocks: the measurement method that uses relative strength as its core input.
Momentum Investing in India: how the idea is practised here, and where it hurts.
Relative Rotation Graphs Explained: the two-axis version of relative strength and its rate of change.
What Is Tracking Error?: the statistical companion measure of how far a portfolio strays from its benchmark.

This article is educational. Altys Labs is not a registered research analyst or investment adviser, and nothing here is investment advice or a recommendation to buy, sell, or hold any security.