What is months on book (MOB)?

Months on book counts the months elapsed since an account was opened or a loan was funded. An account opened in January is at one month on book in February. MOB provides a common age scale for comparing accounts and loan groups originated at different times, and it is a basic field in most retail credit data sets and reports.

MOB is the horizontal axis in vintage analysis, allowing lenders to compare cohorts at the same stage of life. Credit performance typically follows a seasoning pattern, with delinquency and losses rising for some months after origination before leveling off, and the shape depends on the product. MOB is also used as an eligibility rule for actions such as line increases and as an input to behavior scores.

Lenders do not always agree on whether the origination month counts as month zero or month one, so definitions must be consistent across reports. Refinancings, product changes and account transfers can reset or obscure MOB. Comparing portfolios without adjusting for age can lead to wrong conclusions, because a young portfolio may look better simply because it has not yet seasoned.