By analyzing Landau's phenomenologieal thermodynamic potential, we propose a few ways to assess the importance of eighth-power terms and to verify the temperature dependence of the anharmonic coefficients. It is demonstrated that the eighth-power terms should be substantially involved for the adequate description of the thermodynamic behavior of BaTiO₃ around the cubic-tetragonal phase transition. In addition, the temperature dependence of the anharmonic coefficients (at least of the quartic coefficients) is experimentally evidenced to be essential. Both necessities attest to the anomalously strong anharmonicity of this substance.