Director, 10% owner at 6 companies we cover.
24 open-market purchases and 4 sales not made under a pre-set plan, reported from November 30, 2010 to December 15, 2025.
Would copying Gsk PLC’s purchases have beaten the S&P 500?
Buying each purchase the day after it was reported, an equal amount each, and holding it 12 months or until they reported a sale would have turned $10,000 on December 1, 2010 into $1,155 by November 2, 2026, −12.7% a year. The copy held shares only 54% of the time, so the fair comparison is the S&P 500 on those same days with Treasury bills otherwise: $34,348, +8.1% a year (the S&P 500 held throughout made $82,790, +14.2% a year). Beta against the index was 0.63. Twelve months after each report, the average purchase was −16.4% against the S&P 500 (median −22.7%), and 27% beat it: suggestive (p = 0.048 over 8 years), but not enough on its own to rule out luck. Measured from the trade date, the insider’s own timing, the average was −15.8%. After their sales, the shares moved +45.1% against the index over 12 months on average (a negative figure means selling was the right call); 25% trailed it.
With 296 insiders tested, some will look skilled or unskilled by chance alone: when we replaced each insider’s stocks with random companies bought on the same days, about 5.7 cleared the bar ahead and 23.3 behind. One record on its own proves little.
Copying Gsk PLCS&P 500, same days investedS&P 500, held throughout$10,000 copying Gsk PLC’s open-market purchases from December 1, 2010, before costs and taxes, marked weekly. Each purchase gets the same starting stake, bought at the close of the first trading day after the report and sold 12 months later or the day after a reported sale of that stock; uninvested money earns the Treasury bill rate. “Same days invested” holds the S&P 500 only on the days the copy held shares, Treasury bills otherwise.
Return a year, copying
−12.7%
S&P 500, same days invested
+8.1% (held throughout +14.2%)
Time in shares
54%
Sharpe ratio (S&P 500)
−0.09 (0.77)
Biggest drop (S&P 500)
−94% (−34%)
Market sensitivity (beta)
0.63
Purchases copied
23
Trade by trade
Gsk PLC: trades against the S&P 500 afterwards
Measured
Trades counted
Average vs S&P 500
Median
Went the right way
Years
t-statistic
p-value
Evidence
Purchases, 3 months after the report
23
−5.5%
−9.2%
30%
9
−0.71
0.50
No reliable evidence
Purchases, 6 months after the report
23
−14.3%
−14.2%
17%
9
−2.65
0.029
Some evidence: behind
Purchases, 12 months after the report
22
−16.4%
−22.7%
27%
8
−2.40
0.048
Some evidence: behind
Purchases, 12 months after the trade
22
−15.8%
−21.9%
27%
8
−2.38
0.049
Some evidence: behind
Sales, 3 months after the report
4
−3.2%
−2.4%
50%
3
—
—
Not enough years
Sales, 12 months after the report
4
+45.1%
+46.6%
25%
3
—
—
Not enough years
Sales, 12 months after the trade
4
+35.4%
+31.6%
25%
3
—
—
Not enough years
“Average vs S&P 500” is the stock’s return including dividends minus the index’s over the same dates, from the close on the first trading day after the report, or from the close on the trade date.
Only windows that have finished are counted. For sales, a negative figure means the stock lagged after it was sold, so selling was the right call (green).
“Went the right way” is the share of purchases that beat the index, or of sales that trailed it.
The t-statistic uses one average per year of reports, because one person’s trades bunch into the same stock and weeks, and needs trades in at least four different years; the p-value comes from the t distribution with that number of years (with four years, a t of 2 has p = 0.14). “Some evidence” means p under 5%, “strong evidence” p under 1% over at least six years.
Open-market purchases and sales of at least $10,000 in companies we cover, from SEC Form 4 filings; sales made under a pre-set Rule 10b5-1 plan are not shown. “Not yet” means the window has not finished or the share price was under $5 at entry.
These are the shares’ moves after each trade: they say nothing about why Gsk PLC traded and do not suggest the use of non-public information.
How every insider compares · the insider trading backtest.
