Method validation

If suppose RRF not matching againist of Monograph
How to resolve this type of prob

Usp or related ref std not available suppose in house impurity using at that time RRF variation observed

We have received impuritiy from diff Vender but cas no is same eventhough diff RRF observed ?
Different analstys do the analasis at diff ditrs also but same prob observed (hypo perpose plan diff analysts and diff sites)

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