The procedure of the EMD is to construct the upper and lower envelopes of the dataset by spline fitting, and to compute the mean of both envelopes. Then, the dataset is subtracted from the mean, referred to as the sifting process. By repeating the sifting process until the resulting dataset satisfies the aforementioned two conditions, then it is treated as an IMF. The original data is then subtracted from the IMF, and the process
repeated on the remaining dataset to obtain another IMF. The IMF obtained in each cycle by the sifting process involves only one mode of oscillation, with no complex riding waves allowed. As a counterpart to the well-known simple harmonic function, an IMF represents a much more general simple oscillatory mode.