Tense choice in reviewing previous research is subtle and somewhat flexible. The following, therefore, are only generaly guidelines for tense usage. Several studies have shown that at least two-thirds of all citing statements fall into one of these three major patterns:
Jones (1997) investigated the causes of illiteracy.
The causes of illiteracy were investigated by Jones (1997)
The causes of illiteracy have been widely investigated (Jones 1977, Ferrara 2000, Hyon 2004).
There have been several investigations into the causes of illiteracy (Jones 1977, Ferrara 2000, Hyon 2004).
The causes of illiteracy are complex (Jones 1977, Ferrara 2000, Hyon 2004).
Illiteracy appears to have a complex set of causes (Jones 1977, Ferrara 2000, Hyon 2004).