The determination of Earth orientation parameters using techniques which promise an improvement in the precision of their measurements requires a careful evaluation of the systematic errors present in each series of observations. It is now possible to begin to inter-compare two of these sets of observations of the rotation of the Earth (UT1-UTC) obtained routinely and independently. These are the data obtained from laser ranging to the Moon and those derived from the connected-element interferometer information of the U. S. Naval Observatory. Although these data are available for a relatively short time span, it is possible to begin to inter-compare the observations in an effort to derive possible systematic errors existing in the data. This has been done using data from July 1979 to March 1980 in comparison with Bureau International de l’Heure (BIH) data. Using this information, the hypothesis that there is no correlation between the differences of the interferometer and laser ranging data with respect to the BIH must be rejected. This indicates that both sets of observations show that systematic errors do exist in the BIH data. The nature of the correlation of the two series is examined to establish possible models of the systematic errors. A much longer series of data from both techniques is necessary to improve the estimates, and the results will depend critically on adopted models used in the reductions and the degree of smoothing to which the raw data are subjected.