Capital geeks take note – the IRB scaling factor strikes again


Another reminder on the importance of paying attention to the detail courtesy of a story that I picked up reading Matt Levine’s “Money Stuff” column on Bloomberg.

This extract from Matt’s column captures the essential facts:

Here, from Johannes Borgen, is a great little story about bank capital. Yesterday Coventry Building Society, a U.K. bank, announced “a correction to its calculation of risk weighted assets” that will lower its common equity Tier 1 capital ratio from 34.2% to 32.6%. That’s still well over regulatory requirements, so this is not a big deal. But the way Coventry messed up is funny:

“The Society uses Internal Ratings Based (“IRB”) models to calculate its Risk Weighted Assets (“RWAs”) and is seeking to update these models to ensure compliance with upcoming Basel III  reforms. During the process of transitioning models, the Society has identified an omission in connection with its historic calculation of its RWAs. Specifically, the necessary 6% scalar was not applied to the core IRB model outputs. The core IRB models themselves are not impacted.”

For banks that use Internal Ratings Based models, the way the Basel capital rules work is that you apply a complicated formula to calculate the risk weights of your assets, and then at the end of the formula you multiply everything by 1.06. That’s kind of weird. (The Basel capital regime for banks using IRB models “applies a scaling factor in order to broadly maintain the aggregate level of minimum capital requirements, while also providing incentives to adopt the more advanced risk-sensitive approaches.”) It’s weird enough that in the “upcoming Basel III reforms” regulators plan to get rid of it: The 1.06 multiplier is a kludge, and if you measure your risk-weighted assets a bit more accurately and conservatively, you shouldn’t have to multiply them by 1.06 at the end. 

Matt Levine, “Money Stuff”, Bloomberg

For anyone new to this game who wants to dig a bit deeper into how the advanced capital requirements are calculated, the Explanatory Note published by the BCBS in July 2005 is still a good place to start. I published a note on that paper on my blog here. The RBNZ also produced a useful note on how they used the IRB function in the portfolio modelling work they used to support their recent changes to NZ capital requirements.

It should be noted however that none of these documents discuss the 6% scaling factor. I open to alternative perspectives on this but my recollection is that the 6% scaling factor was introduced post July 2005 in one of the multiple recalibration exercises the BCBS employed to ensure that the IRB function did not reduce capital requirements too much relative to the status quo operating under Basel I. It is effectively a “fudge” factor designed to produce a number the BCBS was comfortable with (at that time).


Author: From the Outside

After working in the Australian banking system for close to four decades, I am taking some time out to write and reflect on what I have learned. My primary area of expertise is bank capital management but this blog aims to offer a bank insider's outside perspective on banking, capital, economics, finance and risk.

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