As an observational study it cannot prove cause and effect, but merely point out associations.

The authors cautioned it was an observational study and more work was needed.

He cautioned that more research was needed and that, as an observational study, it only showed an association with the activity.

This keeps the whole data collection process of the observational study entirely unobtrusive.

In an observational study, however, researchers have no control over who receives the treatment.