The sentence "Several studies have been conducted" is correct and usable in written English. You can use it when you are referring to a group of studies that have been completed or undertaken in the past. For example: "Several studies have been conducted on the effects of climate change, and the results show a strong connection between rising global temperatures and increased extreme weather events.".
Several studies have been conducted to model crash severity and investigate the impacts of various factors involved in the crashes.
Several studies have been conducted in this field.
Several studies have been conducted to define the epidemiology of headache in children [6].
Several studies have been conducted to address those questions (Table 2).
Several studies have been conducted to improve pushover methods and overcome these deficiencies.
Several studies have been conducted to define the relationship between velocity and porosity.
Several studies have been conducted on EEG seizure detection[3, 4].
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