APA 6 prefers active voice to passive voice in almost all cases because active voice emphasizes the actor rather than the recipient of the action.
Han shot Gredo.
Gredo was shot by Han.
Here, active voice emphasizes the importance of the person shooting over the person being shot. When it is necessary to focus on the recipient rather than the actor, passive voice is acceptable.
JFK was shot by a mysterious figure in the grassy knoll.
A mysterious figure in the grassy knoll shot JFK.
We work with graduate students every day and know what it takes to get your research approved.