Joseph Snizavich died of brain cancer and his widow blamed his exposure to chemicals at a Rohm & Haas research facility. But proving a connection between the chemicals and cancer could be difficult, if a judge's ruling this week is any indication. Expert opinions must be delivered with a reasonable degree of scientific certainty, said Edward D. Ohlbaum, a professor at Temple’s Beasley School of Law. "We allow experts to do something that nobody else is able to do, and that is to state an opinion," Ohlbaum said.