The process of refining the structure can send the researcher through the main loop many times.  Typically more isotopomers (different mass, but same atoms in the compound) are obtained to help define a wider range of structural parameters.  For each parameter we wish to determine, we need one (or more) set of data.
    The smaller loop of predicting and measuring spectra is perhaps the most important.  The range of microwave frequencies is large compared to the window of frequencies abserved in a single scan.  Therefore; guessing where to start scanning is an important step. Once a spectrum is obtained, only then can the project develop.