That adage is more useful when looking at individual
companies, or maybe new businesses.
For long established, highly regulated operations, I would expect consistent profit margin figures across pretty much all companies in the business to represent a pretty accurate view of the situation, and to show if they are earning extra profit due to COVID.
For long established, highly regulated operations, I would expect consistent profit margin figures across pretty much all companies in the business to represent a pretty accurate view of the situation, and to show if they are earning extra profit due to COVID.