###### Abstract
These authors underlined that the coefficient of variation (CV) was more appropriate in evaluating the dispersion around the mean than SD alone. Hence, they postulated that spectral distributions with nearly the same APE value should have a small value of CV across all considered 50-nm bands to be considered equivalent. The results obtained for the two Spanish sites under scrutiny showed that bijectivity between APE and GTI spectrum shape could be assumed only from a pragmatic point of view and only over the 450-900-nm waveband, where CV stays below 3.3%. The reasons why noticeable dispersion occurred below 450 nm or above 900 nm were deeply analyzed from both an instrumental standpoint and atmospheric physics principles. [PERSON] et al. [33] carried out a similar statistical analysis to that of [PERSON] et al. [19] with spectral GTI data collected on equator-facing surfaces at three Japanese sites for one or two years: Kusatsu (15.3 deg tilt, as in the study discussed above [19]), Tsukuba (latitude 36 deg04' N, 20 deg tilt), and Miyazaki (latitude 31 deg49' N, 35 deg tilt). They concluded that APE uniquely represented the spectral distribution of GTI, even though CV values in excess of 30% were found within the 900-950 nm waveband for the three sites under scrutiny. The literature reviewed above targets flat-plate PV applications only and thus focuses on the _global_ spectrum (on either horizontal or tilted surfaces). The bijectivity between APE and spectral direct normal irradiance (DNI) has not received much attention so far, which is a justification for the present investigation. Apparently, [PERSON] et al. [34] are the only authors who have explored the APE-DNI relationship experimentally and only at a single site. They applied the same general methodology as [PERSON] et al. [19] to spectral DNI measurements collected over a one-year experimental campaign conducted in Kusatsu. [PERSON] and coworkers concluded that a one-to-one relationship did exist between APE and the DNI spectral shape. Unfortunately, they did not analyze the variation of CV across all the studied 10-nm wavebands. It is worth mentioning, though, that a previous theoretical study presented by [PERSON] [35] and based on simulations of the cloudless direct spectrum by means of the Simple Model of the Atmospheric Radiative Transfer of Sunshine (SMARTS) 1 [36] had disproved the bijectivity between APE and the DNI spectral shape. That study showed that different combinations of air mass (AM), aerosol optical depth (AOD), and precipitable water (PW) could result in significantly different DNI spectral distributions with however the same resulting APE value. The present work aims at exploring whether the APE uniquely characterizes the varying DNI spectrum--at least in practical terms--from an experimental standpoint. In view of the above literature review, this study is the second known attempt to elucidate such an issue by analyzing measured DNI spectra. The present 12-month experimental study is based on spectroradiometric measurements at Jaen, Spain. The general methodology of [PERSON] et al. [32] is followed to perform an appropriate statistical analysis of the spectral data.