ITHACA, N.Y. — Most nursing homes had fewer nurses and caretaking staff than they had reported to the government, according to new federal data, bolstering the long-held suspicions of many families that staffing levels were often inadequate.

The records for the first time reveal frequent and significant fluctuations in day-to-day staffing, with particularly large shortfalls on weekends. On the worst-staffed days at an average facility, the new data show, on-duty personnel cared for nearly twice as many residents as they did when the staffing roster was fullest.

The data, analyzed by Kaiser Health News, come from daily payroll records Medicare only recently began gathering and publishing from more than 14,000 nursing homes, as required by the Affordable Care Act of 2010. Medicare previously had been rating each facility’s staffing levels based on the homes’ own unverified…