Mayor Nutter and Police Commissioner Charles H. Ramsey are slated to outline a plan today to prevent corruption within the Philadelphia Police Department. Police and crime experts said devising such a plan would not be easy. There is no way to predict what leads a police officer with a clean background to start breaking rules. Jerry Ratcliffe, a criminal justice professor at Temple, said that in addition to looking at ways to prevent corruption, the department should evaluate how it investigates corruption. "The difficulty for any police department is that you can't actively discipline someone until they've been found to do something wrong," he said. "So it's hard to be proactive."