They are the invisible labourers whose toil in the digital economy powers many rising technology firms.

Crowdworkers pick up the slack where artificial intelligence meets its limits. They do small online data tasks, on an outsourced basis and usually from home, that involve basic computer skills, from labelling images and transcription to identifying porno-graphy, which machines and algorithms alone cannot manage. The work is often repetitive and simple but requires human judgment and insight.

“For a lot of entrepreneurs working on lean start-up budgets, it’s not viable to take somebody on to do this work,” says Lilly Irani, a computer scientist at the University of California, San Diego, who sees the phenomenon as part of the broader socio-economic reconfiguration ushered in by on-demand services such as Uber, Lyft and TaskRabbit.