Internal SMEs and functional employees are absolutely critical during testing and validation because they're the only ones who truly know what "right" looks like for your specific business context. Here's why that matters across each area:

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Solution Validation:

Your SMEs understand the actual business processes, edge cases, workarounds, and unwritten rules that keep operations running. A COTS vendor or implementation consultant knows their product, but they don't know that your procurement process has special approval chains for certain vendors, or that your finance team closes books differently in Q4. SMEs can spot when configured workflows will break real-world processes that weren't fully captured in requirements.

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System Integration:

Functional employees know how data actually flows between systems today—including the informal handoffs, reconciliations, and manual fixes that might not be documented anywhere. They can identify when integrated systems aren't talking to each other correctly, when timing issues will cause problems, or when data transformations lose critical context. They understand the downstream dependencies that generic test scripts might miss.

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Data Conversion:

This is huge. SMEs can look at converted data and immediately spot problems: "Wait, these customer records shouldn't all have the same category" or "These historical balances don't tie to our GL." They understand the business meaning behind the data, know what values are valid or suspicious, and can catch data quality issues that would only surface as operational problems months later. They also know which historical data actually matters versus what can be archived.

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Features and Functionality:

End users will find usability issues and missing capabilities that never surfaced in demos. They'll discover that a five-step process in the new system used to take two clicks, or that critical reports are missing, or that the system can't handle a specific but important transaction type. They think like users, not like testers following scripts.

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The Bottom Line:

External consultants and IT teams can verify that the system works as configured. But only your internal people can verify that it works for your business. They provide the business intelligence that prevents you from going live with a technically functional system that operationally fails.

Plus, involving them builds buy-in, uncovers training needs early, and creates internal champions who'll help drive adoption.