Perhaps you should try some spelling variations:
decrease observed » decay observed (Expand search), case observed (Expand search)
observed objects » observed effects (Expand search), submerged objects (Expand search), buried objects (Expand search)
decrease their » decrease the (Expand search), increase their (Expand search), decreases the (Expand search)
their decrease » the decrease (Expand search), their disease (Expand search), the decreased (Expand search)
decrease hiv » decrease in (Expand search), decreased high (Expand search), decrease with (Expand search)
hiv decrease » via decrease (Expand search), hiv increase (Expand search), will decrease (Expand search)

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

• Removing quotes may allow a broader search: (( decrease their decrease hiv decrease ) OR ( decrease AND decrease observed objects ))~.