The takeaway
Personalis Inc shows a pronounced seasonal pattern over 7 years of data — strongest in December (+15.7%) and softest in March (−11.1%).
Right now
In July, the stock has risen 57% of years, averaging +26.9%, about +24.7 pts better than the S&P 500.
The full picture
Personalis Inc's most dependable month has been December, higher in 5 of 7 years; March has been its least reliable, up just 17% of the time.
| Year | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
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| 2019 | — | — | — | — | — |
Month by month
The stock's clearest edge over the S&P 500 lands in July (+24.7 pts); it has trailed the market most in March (−12.1 pts).
“vs S&P” is Personalis Inc’s average for a month minus the S&P 500’s average for that same month — isolating Personalis Inc’s own seasonal edge from broad market drift.
Reality check
Over the last 5 years, December has closed higher 60% of the time versus 71% across the last 7 years — the pattern is weakening.
Figures are the typical (median) December return and how often it rose — the last 5 years versus the last 7(the heatmap’s default window). This verdict stays anchored to that 7-year window even if you zoom the chart, so it never disagrees with the badges above.
In plain English
Strip the year back and a single month does the heavy lifting: December, up in 5 of 7 Decembers while the other eleven tend to blur together.
Its average (+15.7%) and median (+24.9%) land within a hair of each other — the tell of steady, year-after-year gains rather than one outlier doing the work. That reliability comes with real swings, mind — even December ranges by 27.0% from year to year, so any single year can land far from the average. Crucially, the gain is the stock's own rather than a rising tide's: December has cleared the S&P 500 by +14.7 points above the index. That consistency sets it apart from the field, where the average stock manages December only about 58% of the time.
Only April comes anywhere near it for reliability. At the other end of the calendar, March has been the soft spot — the weakest of 7 months that average a loss (−11.1%), and the edge isn't year-round — the stock has trailed the S&P 500 in March, January, and May. Its roughest month on record was a −35.9% April in 2022 — a reminder of how hard even a seasonal name can fall.
The pattern has softened of late, December's last five years slipping below its longer-run record.
The takeaway is less about when to buy than what to expect: December aside, the stock's months offer little reliable tilt. With a short 7-year record and returns that swing hard year to year, the signal is best held loosely.
Short answers on the stock's best month (December), its worst (March), and whether it really trades seasonally.
Yes, to a pronounced degree. Since 2019 its best month (December, +15.7%) has run well ahead of its worst (March, −11.1%) — the heatmap above shows how steady that gap has been year to year.
December has been the strongest, averaging +15.7% and closing higher in 5 of 7 years since 2019.
It's the weakest, averaging −11.1% — historically a soft spot, though it still varies from year to year.
Explore
These names have the strongest July track records on record — a starting point for comparison.
Before you trade