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You have already proved the first 2 requirements of NIW. Focus on the 3rd one.
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" Please submit any availabe addtional documentary evidence that, as of the September 29, 2005 priority date, you had a degree of influence on your field that distinguishes you from other scientists with comparable academic/professinoal qualifications. The evidence may include, for example, copies of additional published articles that cite or otherwise recognize your research achievments. Similarly, evidence of official recognition conferred on you by government entities or professional organizationa may also be submitted."
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Independent letters will help. However, objective evidence are more important. Please note the officer did not mention independent letters at all. Instead, he/she uses the words like "documentary evidence", "official recognision", "published article". These words clearly suggest objective evidence such as citation, media report, rewards, appreciation letters...
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fsa2000@gmail.com