Teachers who earn advanced certification from the National Board for Professional Teaching Standards are more effective than teachers without that credential, but there is little evidence to show the program has transformed the field in the broader ways its founders envisioned, a long-awaited report released last week by a national scientific panel finds. The Arlington, Virginia-based standards board, created in 1987, has received more than $100 million in federal money to develop and run a system of assessments for recognizing accomplished teachers. As of 2006, 42 states were offering teachers financial incentives for earning the voluntary national certification. Between 2003 and 2007, the program awarded its certification status to more than 63,800 teachers. In the new report, however, a 17-member panel of the National Research Council says it is still unclear whether the process itself leads to better-quality teaching, because too few studies have examined that issue. Beyond the classroom, the panel adds, some research suggests that schools are not yet making full use of the expertise of teachers who qualify for the credential, and that the teachers themselves, once they earn the credential, often move on to schools with more-advantaged student populations.