“R-Factors in Rietveld Analysis: How Good Is Good Enough?”, Toby, B. H. Powder Diffraction 21, no. 1 (March 2006 2006): 67-70. DOI: 10.1154/1.2179804. Open access at https://www.cambridge.org/core/journals/powder-diffraction/article/r-factors-in-rietveld-analysis-how-good-is-good-enough/17439A1F889B689C495549A234D53682 or https://www.researchgate.net/publication/245022853_R_Factors_in_Rietveld_Analysis_How_Good_is_Good_Enough
Apologies for self-citation, but people seem to like this paper.