当发生实体店光顾时，系统会使用自愿参与“Google 意见回馈奖励”的用户构成的样本组验证数据。自愿参与该计划的用户数量是多少？

Google uses a sample group of over 5 million users who have opted into the "Google Opinion Rewards" program to validate store visit data. This large sample size helps ensure the accuracy and reliability of the aggregated store visit metrics. Options like 100,000, 500,000, or 1 million are incorrect as they significantly underestimate the actual number of users in the validation sample, which is a key factor in the robustness of Google's store visit measurement methodology.