The hyperpolarizabilities of organic molecules in solution have been determined using the method of the electric field induced second-harmonic generation (EFISH). In principle, the EFISH technique is capable of obtaining precise hyperpolarizabilities. However, the reported values of the hyperpolarizability for a given compound may vary by an order of magnitude, These variations arise because of the complications with the measurements of the nonlinear optical response, the coherence length, the permittivity, the density, and the refractive-indices of a series of solutions with different concentrations and due to the variations in the external absolute reference, temporal overlap between the electric and laser pulses, choices of solvents, and the choices of the models of the local field factors. We have developed a method to extract accurate relative first hyperpolarizabilities from EFISH data, which eliminates most of the above problems, We plot the square root of the SHG intensity versus concentrations of the material of interest and of p-nitroaniline under identical experimental conditions. It is shown that the ratio of the slope of the sample to that of p-nitroaniline provides a simple and accurate characterization of the first hyperpolarizability of the solute under test. The measurements of the concentration dependent dielectric constant, density, coherence length, and refractive index of solutions are shown to be unimportant in the analysis of EFISH data by this method. The slope ratio method eliminates variability associated with lab-to-lab variations in the absolute reference, the overlap of the electric and optical pulses, and the solvent conductivity.