Berkshire Hathaway is expected to file a quarterly update on its U.S. stock portfolio Thursday afternoon.

The investments Berkshire makes are watched closely because many investors try to copy CEO Warren Buffett's moves.

But it's not always clear who made the investments in the report because it doesn't distinguish between Buffett's stock picks and those made by the company's other investment managers, Todd Combs and Ted Weschler.

Buffett has said that investments smaller than $300 million are likely to be the work of Combs or Weschler who are each managing about $4 billion for Berkshire.

The quarterly report will show Berkshire's holdings at the end of 2012, so some of the information could be out of date.