"main subject of study" is correct and usable in written English.
You can use it when you are referring to a particular topic or field someone is studying. For example, "My cousin is currently studying medicine and anatomy is his main subject of study."
Some primary examples can be classified based on their respective main subject of study: Effects of mental workload on driving performance (including research about how phone conversations or other sources of driver distraction affect driving).
Using the expression of miRNAs to obtain a differential diagnosis in cardiovascular pathologies is the main subject of study for many research groups [ 34].
The aim of this paper is to provide an overview of published studies in which one of the members of the SNOMED family was the main subject of study or plays a secondary role in a study, e.g. to select a patient population for epidemiologic research or to describe an information system in which a SNOMED version was used to code patients.
The fundamental mechanism of the development of "double-diffusive convection" during solidification was one of the main subjects of study in the past.
Since the purpose of this study was to describe the differences in home service use between the sustained or improved group and the deteriorated group in their care needs level, the LTCI service users (n = 624) who continued living in the community for 11 months were enrolled in this study as the main subjects of analysis.
One of their main subjects of discussion is child marriage.
One of the main subjects in electroceramics studies is how to improve or tune one specific electrical property for a defined application.
Site-independent models of evolution have been the main subject of many phylogenetic studies since the inception of the model of Jukes and Cantor [ 12].
The residual limb and prosthetic socket interface was the main subject of interest in previous studies.
E. coli, along with the curved bacterium Caulobacter crescentus, has been a main subject of theoretical and computational studies of bacterial morphogenesis.
Therefore, the main subject of the present work was to study the influence of sparger design and process parameters on the BSD in the sparger region of the considered apparatus.
