WASHINGTON — Cheap coronavirus tests that ordinary Americans can administer at home, without either a prescription or any medical expertise, could significantly drive down infection rates, say researchers from the University of Colorado and Harvard. Their findings counter long-standing concerns that rapid tests are less accurate than those that are sent to laboratories. Those inaccuracies become effectively inconsequential if enough rapid tests are done with sufficient frequency, their statistical models indicate. “The critical finding of our work is that when it comes to screening tests for COVID, turnaround time is absolutely critical,” said Daniel Larremore, a computer scientist at Colorado.