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However , research on the treatment of core symptoms is plagued by limitations in study design , statistical power , and other issues inherent to the study of treatments for autism ( e . g . , heterogeneity of the disorder ) that continue to prevent the elucidation of efficacious treatments .
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
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