The active voice says directly WHO does WHAT. It's concise and clear and exact. Use active verbs to state responsibility for actions and to stress the researcher's role. Scientific publications now ask authors to use the active voice as much as possible.
(active) We analyzed the photosynthetic rate of leaf discs immersed in distilled water. (What did you actually do in this lab? ''We analyzed....'')
(active) All groups found these results. (Who came up with these results? ''All the groups did.'')

You will see a lot of passive voice in scientific writing. It's useful to present actions that would turn out the same no matter who performed them; it concentrates on the result, not on the doer of the action. It's appropriate for referring to standard procedures in general terms:
(passive) When baking powder is added to vinegar, carbon dioxide is produced. (This would happen no matter who performed the action.)

NOTE: The PASSIVE voice is not the same thing as the PAST tense. It describes the relationship of the action to the person doing it, not the time frame: The results will be analyzed is passive because it focusses on what gets analyzed, not who does it. (The active form would be We will analyse the results; that version focusses on the person doing the action.) It was predicted is passive because it avoids saying who did the predicting, not because it refers to the past.