On the rewriting of ESG ratings

One of the major ESG ratings providers rewrites not only the current, but also historical ESG data. And that happens even on a weekly basis.

Actually, many of them change their data. When you take a good look at manuals and documentation, you can see that ESG scores of many data vendors might change if controversies show up.

But are there really controversies for 80% of historical ESG scores that are suddenly uncovered in year 2021 for the period 2011-2017?

You can download the paper at:


One example firm's ESG scores downloaded at 3 different dates:

I guess that you can imagine how all your backtests might change - e.g., if your strategy is based on "ESG score improvement" vs. "ESG score worsening", but many more strategies would be impacted as well. Have a look at the teal and red lines in 2015 and 2016. According to your data download from September 2020, you would have assigned this firm to the "ESG score improvement" group (since 60.5 > 53.6), but according to your data download from November 2020, you would have assigned this firm to the "ESG score worsening" group (since 35.0 < 35.7).