Current market problems and reforms in the underwriting and pricing of subprime loans, including the tightening of underwriting standards by regulators, will have a short-term impact on housing markets, the National Association of Realtors said today. NAR senior vice president and chief economist David Lereah predicted that tighter underwriting practices may cause total home sales to fall by about 100,000 to 250,000 nationally, or no more than 3 percent a year over the next two years.