Operational stock

When industry groups say there are five million robots in factories, they are quoting operational stock. It is not a live census. It is a model: take the installations recorded each year, then subtract the units assumed to have been scrapped or retired after an average service life.

An example: if a country installs 10,000 robots a year for a decade, its operational stock is not 100,000, because arms bought in year one may already be out of service by year ten. The number also depends on how long the statisticians assume a robot lasts, which is why stock figures can be revised.

It matters for reading market news. Installation numbers show what manufacturers bought last year and swing sharply with the economy. Stock numbers show the installed base and move slowly, which is why they are the ones that produce round-number milestones.