The limitations part sums it up, this is a study about real-life microdosing not about microdosing by itself and the lack of substance verification makes it mostly irrelevant to me.
"A key limitation of the present study is the lack of verification of the nature, purity, and dosage of the psychedelic substance used for microdosing. Psilocybin-containing mushrooms were used by 23% of the sample, 14% used legal LSD analogues (such as 1P-LSD), whereas 62% sourced their substance from the black market, mostly LSD (61%). According to the Energy Control's drug checking service (Barcelona), LSD blotter adulteration rates were low during the period when our study was running: in both 2018 and 2019 blotters sold as LSD contained LSD only in 90% (n = 735) of tested samples [personal communication with M. Ventrua from EC, June 2020]. The exact quantity of active ingredient within a given microdose cannot be known with certainty; however, the positive relationship between dose and blind breaking (Figure 4) and that the threshold dose for psychoactivity was consistent with a recent controlled study (12 µg vs 13 µg; Bershad et al., 2019a) provide some reassurance. Nonetheless, our results should be not understood as clinical evidence, rather they are representative of ‘real life microdosing’."
https://elifesciences.org/articles/62878#s4-1
"A key limitation of the present study is the lack of verification of the nature, purity, and dosage of the psychedelic substance used for microdosing. Psilocybin-containing mushrooms were used by 23% of the sample, 14% used legal LSD analogues (such as 1P-LSD), whereas 62% sourced their substance from the black market, mostly LSD (61%). According to the Energy Control's drug checking service (Barcelona), LSD blotter adulteration rates were low during the period when our study was running: in both 2018 and 2019 blotters sold as LSD contained LSD only in 90% (n = 735) of tested samples [personal communication with M. Ventrua from EC, June 2020]. The exact quantity of active ingredient within a given microdose cannot be known with certainty; however, the positive relationship between dose and blind breaking (Figure 4) and that the threshold dose for psychoactivity was consistent with a recent controlled study (12 µg vs 13 µg; Bershad et al., 2019a) provide some reassurance. Nonetheless, our results should be not understood as clinical evidence, rather they are representative of ‘real life microdosing’."