For the pressure-volume study what experimental factors are assumed to be kept constant

The behavior of gases is a function of pressure (P), volume (V), temperature (T), and the number of moles (n) of the gas. This is reflected by the simplest correlation for the gases which is the ideal gas law (PV=nRT with R as the universal gas constant). For the experimental results to reflect ONLY the effect of pressure and volume, the other factors that could affect the behavior of the gas, particularly the temperature and the number of moles of the gas, must be kept constant. 

The answer is reflection.