Title: The TruEnd-procedure: Treating trailing zero-valued balances in credit data

Abstract: A novel procedure is presented for finding the true but latent endpoints within the repayment histories of individual loans. The monthly observations beyond these true endpoints are false, largely due to operational failures that delay account closure, thereby corrupting some loans in the dataset with `false' observations. Detecting these false observations is difficult at scale since each affected loan history might have a different sequence of zero (or very small) month-end balances that persist towards the end. Identifying these trails of diminutive balances would require an exact definition of a "small balance", which can be found using our so-called TruEnd-procedure. We demonstrate this procedure and isolate the ideal small-balance definition using residential mortgages from a large South African bank. Evidently, corrupted loans are remarkably prevalent and have excess histories that are surprisingly long, which ruin the timing of certain risk events and compromise any subsequent time-to-event model such as survival analysis. Excess histories can be discarded using the ideal small-balance definition, which demonstrably improves the accuracy of both the predicted timing and severity of risk events, without materially impacting the monetary value of the portfolio. The resulting estimates of credit losses are lower and less biased, which augurs well for raising accurate credit impairments under the IFRS 9 accounting standard. Our work therefore addresses a pernicious data error, which highlights the pivotal role of data preparation in producing credible forecasts of credit risk.