Which product marketing metric did the UK Government Digital Service identify as most crucial for measuring the success of their 'GOV.UK' platform launch?

Government agencies worldwide have been investing in digital transformation to improve service delivery to citizens. This has involved significant changes in their product design approaches and marketing strategies to ensure adoption of new digital services. How well do you know the key metrics and strategies that have defined successful government digital transformation initiatives?

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Task completion rate (citizens' ability to successfully complete intended government service tasks)
User acquisition cost (cost per new citizen registering for digital services)
Digital service uptime percentage (system reliability measurement)
Cross-department referral rates (citizens moving between different government services)