The cosmic ray intensity data from Rome neutron monitoring station has been harmonically analyzed for the period 1976-2004 covering three solar cycles 21, 22 and 23. This analysis is performed for four different groups of days 60QD, 120QD, CQD and AD with statistical errors. During the entire period of investigation, the amplitude of higher harmonics is low at / near the minima of each solar activity cycle for all groups of days. During the maxima of SAC-21 & 23, the phase of semi – diurnal anisotropy is shifted to earlier hours for 60QD, 120QD and AD, which confirms the 22-year periodicity of semi-diurnal anisotropy. Statistical errors in case of AD for semi-diurnal as well as tri-diurnal anisotropy have been observed to be low in comparison to the different groups of quiet days.