THE THREAT of homelessness hangs over many heads. Nearly 5 percent of households nationally have "worst case housing needs." That means they pay more than 50 percent of their income for housing, have no housing assistance, and have low incomes, according to a new report from the Boston Foundation and the McCormack Graduate School at the University of Massachusetts in Boston.

In Massachusetts, high housing costs heighten the threat. Lose an apartment and it can take thousands of dollars in rent and deposits to get a new one. Homeless shelters have become a safety net, but they're an expensive alternative. On average a three-month stay in a shelter costs $2,969 for individuals and $8,820 for families, according to the report.

Helping people avoid evictions and foreclosures in the first place is clearly a better choice. To measure the state's prevention efforts, the report looks at three programs. The RAFT program (for Residential Assistance for Families in Transition) gives low-income families grants to pay for needs such as the first month's rent or overdue utility bills. The other two programs are funded by foundations. The Homelessness Prevention Initiative awarded grants to 19 organizations to test ways to help different populations, including people leaving prison and those with mental illness. "RAFT Plus" is modeled after RAFT but helps families who are ineligible for state help, in some cases because they earn too much.

The report's key finding is that prevention works, and is much cheaper than using shelters. Average costs ran from $456 to $1,707 per case. And 12 months after families got help, the majority were still housed. The success rates ran from 63 percent for individuals who got foundation-funded help, 79 percent for those who got state help, and 91 percent for foundation-funded RAFT Plus recipients.

Among the successful strategies were home visits, and help crafting household budgets, searching for housing, and navigating housing court. The report also shows weak spots in well-meaning programs. Prevention was less successful for runaways, families escaping domestic violence, people leaving prison, and Hispanic families with multiple risks such as low incomes, little education, and overcrowded households.

The findings should be crafted into a public plan to substantially increase prevention efforts. State government and foundations should also run pilot programs to study new ways to help former convicts and others with complicated needs. Prevention should be a program staple. It's a far cheaper way to spare people from the trauma of homelessness. And now the data are there to prove it.