VCG Public Project, explained.
The Vickrey–Clarke–Groves approach selects a reported-surplus-maximising outcome and charges each participant the externality imposed on others under the chosen payment rule.
Why it happens
A shared project costs 60. Reported benefits are your bid, 35 and 10; build when their sum reaches 60. If built, each pivot payment is max(0,60 minus the other reports). Your true benefit can differ from your report. Inspect utility and the shortfall between payments and cost.
The Vickrey–Clarke–Groves approach selects a reported-surplus-maximising outcome and charges each participant the externality imposed on others under the chosen payment rule.
Read the result
Separate allocation, incentives and budget balance. A project can maximise reported surplus and still collect far less than its cost.
A worked example
A project worth more than its collected payments
Keep true benefit and report at forty-five. With other reports thirty-five and ten, the total ninety exceeds the cost sixty.
Run the mechanism: payments are fifteen, five and zero, leaving a forty-unit funding shortfall despite positive reported total surplus.
Lower your true benefit to five and compare truthful reporting with an exaggerated report that triggers construction and a fifteen-unit payment.
OPTIONAL DEEPER DETAILGo deeper: inside the model
Inside this model
A shared project costs 60. Reported benefits are your bid, 35 and 10; build when their sum reaches 60. If built, each pivot payment is max(0,60 minus the other reports). Your true benefit can differ from your report. Inspect utility and the shortfall between payments and cost.
Where this idea is useful
A practical use
A mechanism can encourage truthful benefit reports while still requiring an external source of funding.
A common misconception
“A truthful mechanism must pay for itself.”
Truthfulness, efficiency and budget balance are separate properties. This pivot mechanism assumes outside funding for its shortfall rather than secretly removing the project cost.
What this explanation leaves out
- The model assumes quasi-linear utility, nonnegative values, no collusion and an outside budget. Truthfulness is weak dominance under these rules, not a guarantee of universal fairness or budget balance.
Why is your pivotal payment fifteen when the project is built?
The other two report forty-five together, fifteen short of the sixty cost. Your participation makes the project possible and imposes that externality under this rule.
Which mechanism property are you trying to achieve, and who finances any gap left by its payment rule?
Associated thinkers
Associations marked provisional are awaiting source review.
Further reading
Explore the original research or the teaching reference behind this experiment.