In hundreds of police departments across the country, the percentage of whites on the force is more than 30 percentage points higher than in the communities they serve, according to an analysis of a government survey of police departments. Minorities make up a quarter of police forces, according to the 2007 survey, the most recent comprehensive data available. Experts say that diversity in the police force increases a department’s credibility with its community. “Even if police officers of whatever race enforce the law in relatively the same way, there is a huge image problem with a department that is so out of sync with the racial composition of the local population,” said Ronald Weitzer, a sociologist at George Washington University. Listed below are local police departments from 15 metropolitan areas, sorted so that departments with the largest percentage-point differences of white officers to white residents are at the top.