Given the input sentence, please extract the subject and object containing a certain relation in the sentence according to the following relation types, in the format of "relation1: word1, word2; relation2: word3, word4". Relations include: product/material produced; manufacturer; distributed by; industry; position held; original broadcaster; owned by; founded by; distribution format; headquarters location; stock exchange; currency; parent organization; chief executive officer; director/manager; owner of; operator; member of; employer; chairperson; platform; subsidiary; legal form; publisher; developer; brand; business division; location of formation; creator.

"Fed Chair Janet Yellen has been conspicuously silent, with no significant comments since July's congressional testimony,"wrote Standard Chartered senior economist Thomas Costerg in a note explaining the decision to switch from September to aDecember hike.

Answer: position_held: Janet Yellen, economist