A systematic study on the anharmonicity of phonons is made for single-crystal type-I clathrates: n-type Ba8Ga16Ge30 (n-BGG), p-type Ba8Ga16Ge30 (p-BGG), n-type Sr8Ga16Ge30 (n-SGG), n-type K8Ga16Sn30 (n-KGSn), and n-type Ba8Ga16Sn30 (n-BGSn), based on their heat capacity C-p at low temperatures (T) down to 360 mK. The low-T linear terms (obs)gamma T of C-p, including the tunneling term of the atoms accommodated in the host cages (gamma T-ph = alpha T) and the Sommerfeld itinerant-electron term (gamma T-e), are successfully separated through careful measurements of single crystals with various carrier concentrations. The values of the minimum density of anharmonic potentials are deduced from a to be 0.12 +/- 0.24 for n-KGSn, 0.47 +/- 0.24 for n-BGG, 1.9 +/- 0.8 for p-BGG, 6.0 +/- 0.9 for n-SGG, and 10.9 +/- 0.7 for n-BGSn in units of x 10(15) cm(-3). The effective mass (m*/ m(0)) is determined from gamma(e) to be 1.01 +/- 0.25 for n-BGG, 1.20 +/- 0.19 for p-BGG, 1.68 +/- 0.24 for n-SGG, 1.86 +/- 0.54 for n-KGSn, and 2.05 +/- 0.48 for n-BGSn in units of free electron mass m(0), and the electron-phonon interaction strength can be evaluated from these values. It is shown that both the thermal conductivity (kappa) and the electron-phonon interaction strength (lambda) agree well with the a parameters deduced from C-p. The differences in kappa known between n- and p-BGGs are ascribed to the influence of defects at the crystallographic 6c sites, which are clearly indicated by magnetic susceptibility measurements. This is very different from the situation in the other clathrates.