The phrase "are the main variables" is correct and usable in written English.
It can be used when discussing key factors or elements in a particular context, such as research, analysis, or data interpretation.
Example: "In this study, the factors that influence the outcome are the main variables we need to consider."
Alternatives: "constitute the primary factors" or "represent the key elements".
These are the main variables of interest and the coefficients γ1 and γ2 are referred to as the treatment effects of contracts and cooperatives respectively.
Two are the main variables to monitor in a participatory process aimed at consensus building: 1) the degree of consensus and 2) the utility perceived by the agents.
The number and type of plies (full or partial wrapping), the type of loading (monotonic and cyclic actions) and the type of fiber (basalt and carbon) are the main variables investigated.
Age and obesity are the main demographic characteristics related to prolonged MV, and history of COPD, PaO2/FiO2 at intubation, lung injury severity score (LIS) [43] and SOFA [28] at admission, are the main variables related to prolonged MV.
The result of the past study in Tabriz air pollution indicates that the metrological parameters and, especially, wind blowing are the main variables in intensification and alteration of Tabriz air pollution (Sadr Mousavi and Rahimi 2008, 2010, Mojtabazadeh 2005).
Maps of efficiency of gas separation were plotted and showed that liquid flow rate, inclination angle and pressure difference between casing and production pipe outlet are the main variables related to the gas separation phenomenon.
(Customs duties are the main variable).
Degree of dependency and the formal care received were the main variables that explained the variability of informal caregiving time provided.
The amount of PPZ and the film thickeners were the main variables affecting film mechanical properties.
The longitudinal reinforcement ratio and configuration of stirrups were the main variables investigated in the experimental programme.
Reaction temperature, feed flow rate (Q), iC4/H2 ratio and the catalyst type were the main variables that were investigated.
