Which retail innovation, first widely implemented in 1916, revolutionized shopping by allowing customers to select their own merchandise without clerk assistance?

The history of retail has seen dramatic transformations that shaped how we shop today. From ancient marketplaces to modern shopping experiences, retail innovations have reflected changing societies and consumer needs throughout human history. This trivia question focuses on a pivotal development in retail operations that fundamentally changed the shopping experience in the early 20th century.

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Self-service grocery stores (Piggly Wiggly model)
Department store catalogs with mail ordering
Coin-operated vending machines
Installment payment plans for large purchases