The 5th edition of the Publication Manual of the American Psychological Association (APA) has this to say on page 33:
For literature review and
description of procedure (if of past
events): use the past tense ("Jiang showed")
or present perfect ("Jiang has
For describing results: use the past tense
(e.g., "weight increased
For discussing results and
presenting conclusions: use the present
tense (e.g., "The results of this
According to the APA, using the present tense for (3) includes the readers in "deliberating the matter at hand" (Publication manual, APA, p. 33). In other words, readers feel as if they are part of the final process of arriving at the all-important conclusion.
More on tenses can be found on pp. 41-44. Areas discussed include:
preferring the active voice over the
passive (e.g., "Jiang designed the
experiment" NOT "The experiment was designed by Jiang").
Note, however, that the passive is acceptable
in some situations; the APA gives
expository writing as an example and
in instances where you intend to
emphasise the "object or recipient
of the action" (APA, p. 42). You might also want to use the passive to soften the impact of sentence after sentence written in the active
using the past tense for "specific, definite"
(APA, p. 42) situations in the past
(e.g., "Jiang (2010) presented similar findings.").
- using the present perfect tense for expressing
the same situations in the point
above that are neither specific nor
definite (APA p.43)(no example of this from me) or for describing "an action beginning in the past and continuing to the present" (APA, p. 43), (e.g., "Since the publication of the experiment, researchers have attempted to disprove its results").
- using the subjunctive (best that you read
it for yourself, in fact, you should
try to get hold of a copy of the
APA's manual and go over the various sections. That would be
better than anything I could summarize here for you.)
APA's website is www.apastyle.org
Disclaimer: All examples are mine.
Hope this helps