I want to know the answer to the following question:
What method did manufacturers use much of the time to compare film speeds?
All the background I'm provided with is that "Still, it remained difficult for manufactures to reliably determine film speeds, often only by comparing with competing products, so that an increasing number of modified semi-Scheiner-based systems started to spread, which no longer followed Scheiner's original procedures and thereby defeated the idea of comparability.". Is that enough to answer the question?