Subtract the Sector's Move to See What the Stock Did

The headline says the stock jumped 4% after its product launch. Open the sector ETF in the next tab. It’s up 3.5%.

Most of that jump came from the sector. The launch earned about half a point.

One day, three numbers

Here’s a hypothetical software stock on a day the whole market rallied.

The market added two points to the day. The sector, rallying harder, added another point and a half on top of that, and once you take both out, the company’s own news moved the stock by half a percent, which is small enough to be noise on a day with moves this size.

That’s a very different story from “shares jump 4% on launch.”

It matters for how you act. A trader reading the 4% might buy the next morning. The launch looks like it’s working. A trader who saw the 0.5% would conclude the market barely noticed the launch at all, and that the next day’s move depends mostly on what the sector does.

The subtraction runs the other way too. A stock that closes flat on a day its sector fell 3% had a strong session, three points better than its group, and a quote page showing 0.0% in gray will never tell you that.

Earnings days are where this pays off most. The morning after a report, the stock’s move mixes the market’s mood, the sector’s mood and the reaction to the numbers. Only the last one tells you what investors thought of the quarter.

An aside on beta

The straight subtraction assumes the stock moves one-for-one with its benchmark. Many don’t.

Beta is the stock’s historical sensitivity to the market. Quote pages usually measure it against a broad index. Say this stock’s quote page lists a beta of 1.5.

A different benchmark, a different answer. Against the sector, half a point. Against the market with beta, a full point.

Beta comes from past prices and drifts over time. A sector ETF, meanwhile, catches industry news that a market beta misses. For a single-day reaction to company news, the sector subtraction is usually the cleaner read, and beta is a useful cross-check when the stock doesn’t have a close sector match.

Stretch it to a week or a month

The same logic works over longer windows, and it’s often more useful there, because a single day is noisy and a month shows whether the stock has been leading or lagging its group.

Up 6% in a month sounds fine. It lagged its sector by about two points.

Over a day, subtracting percentages is close enough. Over longer periods, the ratio version is more precise because returns compound. The two answers stay close unless the moves are large. Either tells you the same thing here: the stock has trailed its peers, and whatever you believe about the company, buyers have preferred the rest of the group.

Getting the numbers off a quote page

For one day, you need two percentages. Every quote page shows the day’s change. The sector ETF has one too.

For longer windows, use a comparison chart. Most charting tools let you add a second ticker to a price chart, which switches the scale to percentage change from the start date, so put the stock and its sector ETF on the same chart, set the start to the beginning of the period you care about, and read both lines at the right edge.

Pick the ETF with care. Check its fact sheet for the top holdings. Companies from another corner of the industry make a poor benchmark. The walkthrough on comparing a stock to its sector covers choosing one.

Check the market too. On a day like the one in the narrow heatmap rally example, the index can say up while most stocks fell, and the sector number helps you see which of those days you’re looking at.

When the stock is the sector

The subtraction breaks when the stock is a large part of its own benchmark.

Take a hypothetical sector ETF where one company makes up 20% of the fund. That company rises 5% and every other holding is flat. The ETF rises 1%, all of it from the one stock.

Subtract the sector and you get 4%. That’s too low. Part of what you subtracted was the stock itself.

The biggest companies in any sector fund run into this. You can spot it on the fund’s fact sheet, which lists each top holding with its weight, and if the stock you’re studying sits near the top of that list with a double-digit share, the ETF is partly a copy of the stock. For those, use an equal-weight version of the sector fund if one exists. Or average a handful of direct competitors by hand. Both give you a benchmark the stock can’t drag along with it.

For smaller companies with a well-matched ETF, subtracting the sector takes a few seconds on any quote page, and it keeps you from mistaking a rising tide for company news, which is an easy way for a good headline to turn into a bad entry.