An equation of state of a fluid can usually be expressed in the form f₁(V, T) = P. When the reduced form of the equation exists, it may be expressed as f₂(v, t) = p, where p, v, and t are the reduced values of P, V and T. Since the critical volume cannot be measured with precision, the recent fashion is to use, instead of v, the accurately obtainable pseudo-reduced volume φ, leading to a reduced equation f₃(φ, t) = p. It is now suggested that a calculation and plot of the quantity Y = 100[f₃(φ, t)/p - 1] against X = p (t constant) or X = t (p constant), using experimental values of p, φ, and t for some real fluid, will reveal in a quantitative manner the discrepancy between the experimental behaviour of that fluid, and of a fluid obeying the particular equation of state under consideration. A set of graphs at various constant temperatures or pressures may be drawn.