Private equity firms own only a small percentage of US hospitals, but they’ve been working to increase their share for two decades. They’ve pooled money from institutional investors and individuals to build investment funds that buy up hospitals and related health care entities, often intending to sell them after a few years. Whether the change in ownership has any effect on patient care hasn’t much been documented, the New York Times reports. Now, researchers have assembled concerning evidence. In the first three years under equity firm ownership, serious problems among Medicare patients incl…