Some of the groups at risk for no credit or a thin file include young adults, the elderly (if they haven't used credit in awhile), new immigrants and people who avoid credit.

Working to establish credit? Go slow, be very selective in your applications and nurture existing accounts. Never pay late, and -- with credit cards -- keep balances reasonable.

With a thin file, even small mistakes can make a big difference in your score, says Ethan Dornhelm, principal scientist with FICO, which developed the FICO score.

Particularly dangerous: multiple applications, he says.

Someone who is "exploding onto the credit scene" -- going from no credit to multiple applications in a short period -- is "someone who could be getting in over their head," he says.