On a Thursday in early September, more than 40 strangers logged in to Instacart, the grocery-shopping app, to buy eggs and test a hypothesis.

Connected by videoconference, they simultaneously selected the same store — a Safeway in Washington, D.C. — and the same brand of eggs. They all chose pickup rather than delivery.

The only difference was the price they were offered: $3.99 for a couple of lucky shoppers. $4.59 or $4.69 for others. And a few saw a price of $4.79 — 20% more than some others, for the exact same product.

The shoppers were volunteers, participating in a study published Tuesday and organized by the Groundwork Collaborative, a progressive policy group, and Consumer Reports, a nonprofit consumer publication. In tests in four cities across the country, nearly 200 volunteers

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