Different sampling strategies explore regions in compositional space where it is easier or harder to find discordant examples, but the trends should hold over all conditions when the data is combined. Indeed, statistically significant differences in reaction outcome were observed for all four systems studied. Water promotes crystal formation (N+-> N-+) for 4-methoxyammonium and isopropylammonium systems, consistent with expectations from prior ITC results. In contrast, water inhibits crystal formation (N-+ > N+-) for dimethylammonium and acetamidinium systems, contrary to previous expectations. The interventional outcomes agree with the qualitative predictions in Figure in 3 of the 4 tested systems, despite the wildly different sampling strategies employed in the historical dataset. Furthermore, the fewest discrepancies are observed for iso-propylammonium in these interventional experiments, consistent with the difficulty estimate provided by the contour lines in Figure . Discrepant reaction outcomes are scattered throughout compositional space in each system (see Figures ), supporting the first physical interpretation of the statistical model. To explore the relevance for device fabrication, perovskite thin films were fabricated via spin-coating for each of the four cations using the Soltrain system. Separate films were prepared in a glovebox using precursor solutions prepared without water, 1% v/v water, and 2 % v/v water. The same solvents and temperature ranges were used as the ITC counterparts. The grain length distribution of the resulting films was characterized by scanning electron microscopy (SEM). X-Ray diffraction (XRD) patterns indicate that the single-crystal and thinfilm syntheses resulted in the same perovskite phase (see Supporting Materials for relevant experimental details and XRD comparisons). Water has only a slight effect on the grain lengths of 4-methoxyphenylammonium and acetaminidium (see Figure ). In contrast, water increases grain length in iso-propylammonium perovskite thin films and decreases grain length in dimethylammonium films. The latter is unusual, as most prior studies report water enhancing grain growth, but it is consistent with the corresponding ITC results. In spin-coating experiments, larger grains occur when grain growth is faster than nucleation (typically at lower supersaturation of the perovskite precursors) and smaller grains occur when nucleation is faster than grain growth (typically at higher supersaturation). Water promotes nucleation of iso-