A cluster buy is when multiple corporate insiders at the same company purchase its stock on the open market within a short period — typically a few days to a couple of weeks. A single insider buying is interesting; several buying independently around the same time is a much harder pattern to explain away, which is why researchers and investors watch cluster buys closely.
Why the pattern matters
Insiders sell for all sorts of routine reasons, but they generally buy for one: they think the stock is undervalued. When a CEO, a CFO, and two directors all reach that conclusion at nearly the same moment and back it with their own money, the combined signal is stronger than any one purchase. Academic studies of insider trading have repeatedly found that clusters of open-market purchases tend to precede better subsequent returns than isolated trades.
What counts as a genuine cluster
Not every group of filings is a true cluster. The purchases should be: made on the open market (transaction code P, not grants or option exercises); by different individuals, not one person filing several lots; and close together in time. A cluster of small token purchases means less than a cluster of large, conviction-sized ones. Reading the transaction codes is what separates a real cluster from coincidental activity.
How to use cluster-buy data
Cluster buys are best treated as a starting point for research, not a buy signal on their own. Once you spot one, look at the size of the purchases relative to each insider's existing stake, whether the buyers include the most senior executives, and what was happening at the company at the time. Our cluster-buys screener surfaces these automatically so you can review the strongest ones.
Related: What is a Form 4? · How to read insider trades · Browse recent cluster buys.