Tense usage when referring to previous research

The beginning of a literature review generally starts with a sentence summarizing previous research. Use of an incorrect verb tense is a very common mistake.

Which is the correct verb tense?

Can you explain the difference?

1. Various studies examined the impact of Covid-19 on educational inequality.
2. Various studies have examined the impact of Covid-19 on educational inequality.

When speaking generally about previous research, the present perfect is generally used. Thus number 2 is the correct answer. The reason is that the present perfect is used to link the past with the present. In sentence 2, have examined means the studies until the present.

The simple past is used for finished actions in the past so sentence 1 would mean specific studies done in the past. An English speaker would most likely be wondering which studies and when they were done. A citation would commonly be used.

Referring to previous research

These are general guidelines. The difference is not always completely black and white.

present perfect tense: used when writing about previous research in general; the research to the present

simple past tense: used when writing about specific research; often used with a citation

The examples below are from the Manchester Academic Phrasebank, an excellent resource for generic phrases.

present perfect tense

A large and growing body of literature has investigated…(the body of literature until the present)

The academic literature on X has revealed the emergence of several contrasting themes. (the literature until the present)

Only in the past ten years have studies of X directly addressed…(from ten years ago until now)

A great deal of previous research into X has focused on…(the research to date)

simple past tense

Smith (1999) found that as levels of literacy and education of the population rise… (a specific study done in the past)

In an investigation into X, Smith et al. (2012) found… (a specific investigation done in the past)

The first serious discussions and analyses of X emerged during the 1970s with… (studies done in the past)