Simplify the following text for a Grade 6 native English-speaking person.
Original: UBS Wealth Management Research has expanded the idea of the Big Mac index to include the amount of time that an average worker in a given country must work to earn enough to buy a Big Mac . The working-time based Big Mac index might give a more realistic view of the purchasing power of the average worker , as it takes into account more factors , such as local wages .
Simplified: The Big Mac Index is an economic index .