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Those who study contemporary language changes , such as William Labov , acknowledge that even a systematic sound change is applied at first unsystematically , with the percentage of its occurrence in a person 's speech dependent on various social factors .
 William Labov
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 William Labov