The sentence "Various studies have been conducted" is correct and usable in written English. You can use it when discussing research or a situation in which multiple studies have been conducted in order to support a point or argument. For example, "Various studies have been conducted on the impact of climate change on biodiversity, and the results show that its effects are far-reaching and detrimental.".
Various studies have been conducted to study the morphological changes seen in NIU employing OCT.
Various studies have been conducted on the aspects of entrepreneurship capabilities.
Internationally, various studies have been conducted on the effects of metacognitive intervention on learning approaches.
Various studies have been conducted in this regard, and various papers have been presented.
Even various studies had been conducted in studying the bond and anchorage in conventional RC but the applications of these codes of practice are unsuitable for HSC.
Several studies have been conducted in various clinical settings on the accuracy and utility of IGRA and these have been reviewed elsewhere [5].
Over the last several decades, various research studies have been conducted with the purpose of quantifying and validating flow properties and transport mechanisms.
Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.
Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com