Most leading textbook companies publish a new edition every 3 or 4 years, more frequently in math and science. Harvard economics chair James K. Stock has stated that new editions are often not about significant improvements to the content. "New editions are to a considerable extent simply another tool used by publishers and textbook authors to maintain their revenue stream, that is, to keep up prices." A study conducted by The Student PIRGs found that a new edition costs 12% more than a new copy of the previous edition (not surprising if the old version is obsolete), and 58% more than a used copy of the previous edition. Textbook publishers maintain these new editions are driven by demand from teachers. That study found that 76% of teachers said new editions were justified “half of the time or less” and 40% said they were justified “rarely” or “never”. The PIRG study has been criticized by publishers, who argue that the report contains factual inaccuracies regarding the annual average cost of textbooks per student.