[pcabc]

I'd have a look at some individual charts to confirm the data, as from a quick visual check of 2020 for example, I'm seeing more stocks above their Mansfield RS zero line than not when looking at the Nasdaq 100 stocks for example. This is just a quick visual check mind. But the Percentage above their Mansfield RS zero line in your chart seems far too low.

The simple answer here is to run the calculations on the Nasdaq 100.  This will take some time, but not as much as the Russell 3000 or UK pools.  If the Nasdaq 100 plots correlate with a reasonable sample of NASDAQ 100 stocks then the math are likely good.

I've plotted the Nasdaq 100 versus the Nasdaq Composite below and it indicates that the 100 was stronger than the Composite for most of 2020, but weakened October onwards.  So there may have been a bias towards a the stronger stocks within the 100 for Jan - Oct. That might also apply _within_ the Nasdaq 100.  But I really need to run the calcs for the Nasdaq 100 as checking is easier.