What is an Expert Judgment?
Expert judgment means consulting people with relevant specialized knowledge — team members, consultants, other PMs, technical authorities — and it appears as a technique in nearly every PMBOK process for a blunt reason: most project decisions outrun the data available, and experience fills the gap.
Use it with eyes open: experts carry biases (anchoring, optimism, their last project's scars), which is why structured methods like Delphi and nominal group technique exist to harvest expertise while filtering its distortions.
Worked example
Estimating dewatering for a riverside excavation, the PM's spreadsheet says two pumps. The geotechnical engineer who's built on this river twice says four, plus standby — "the gravel lens at six meters always makes a liar of the borings." The project takes the judgment over the calculation. In week 3, the gravel lens appears, exactly as advertised.