Perhaps you should try some spelling variations:
by preprocessing » the preprocessing (Expand search), and preprocessing (Expand search), by processing (Expand search)
covid decreased » corn decreased (Expand search), showed decreased (Expand search)
decrease covid » increase covid (Expand search), increased covid (Expand search), disease covid (Expand search)
decreased by » decreased in (Expand search), increased by (Expand search), decreased il (Expand search)
decrease and » decreases and (Expand search), decrease in (Expand search), decrease of (Expand search)
and decrease » and decreases (Expand search), and decreased (Expand search), and increase (Expand search)
decrease on » decrease in (Expand search), decrease of (Expand search), decrease or (Expand search)
on decrease » on decreased (Expand search), of decrease (Expand search), or decrease (Expand search)

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: (( decrease covid decreased by preprocessing ) OR ( decrease on decrease AND decrease ))~.
• The words AND, OR and NOT may confuse the search; try adding quotes: "(( decrease covid decreased by preprocessing ) OR ( decrease on decrease and decrease ))~".
• Adding a wildcard symbol may retrieve word variants: (( decrease covid decreased by preprocessing ) OR ( decrease on decrease and decrease ))*.