Left panel: histogram of the differences (in nHz) in the estimated central frequencies between the 2088 day GONG data set using the m-averaged spectrum technique and the coeval 2088 day GONG Korzennik (2005)'s estimates. Right panel: histogram of the differences (in nHz) in the estimated frequencies between the 2088 day GONG and MDI data sets using the m-averaged spectrum technique in the sense GONG minus MDI. The corresponding Gaussian function fits and their associated mean values and standard deviations are also indicated.