Consistency in the use of tenses

It is important to use tenses consistently unless there is a good reason for a change. For example, in the sentences below there is a shift in tense, but this is justified because, while the first sentence refers to past time (yesterday), the second and third sentences refer to habitual behaviour, for which the present tense is used.

He missed the lecture yesterday. That’s not unusual, though. He only attends once in a while.

In the following extract, however, the change in tense in the last sentence is not appropriate because the action being described also happened in the past:

The data for the study were collected over a three-week period. First, a questionnaire was distributed to all participants. Then, semi-structured interviews are conducted with eight of the participants to explore emerging issues.

The correct tense in the last sentence is therefore the simple past:

The data for the study were collected over a three-week period. First, a questionnaire was distributed to all participants. Then, semi-structured interviews were conducted with eight of the participants to explore emerging issues.

Careful proofreading will help you to avoid this type of mistake.