Shares of energy firms in the index declined 1.6% as oil prices slumped (source)
I somehow feel like there should be a preposition after "declined" marking the degree of the decline. Shouldn't it be "declined by 1.6%"? For example, I would write:
Stocks declined by 20%.
Google seems to suggest that both are common. However, it also seems that some of the search results are headlines or false positives, which makes it hard to tell which one is correct/idiomatic. Is the preposition "by" needed/optional? Is there a difference in meaning?
Stocks fell 20%
Stocks fell by 20%