A cluster is several insiders at the same company buying within a short window of each other. It is treated as more informative than a single purchase, and the reasoning is about eliminating alternative explanations rather than about the money involved.
One director buying can mean many things. They may have had a bonus to deploy, or a personal view unrelated to the business, or a habit of buying every year regardless. Those explanations are individual. When four people who all sit in the same meetings independently reach the same conclusion in the same fortnight, the individual explanations stop working and a shared one — that they collectively think the shares are cheap — becomes the simpler reading.
Size is not the same as breadth, and breadth is the part that's hard to fake. A single very large purchase by a founder who already owns a third of the company tells you less than four modest purchases by four unrelated executives, because the founder was always going to be exposed either way.
Clusters are also partly an artefact of the calendar: closed periods mean everyone's dealing window opens at the same moment, so some clustering is coincidence of timing rather than a meeting of minds. That's why the window matters, and why a cluster spread across an open period reads differently from one bunched in the first two days after results.
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Information only, not investment advice.