The [PCA plot of the barley data](https://aml4td.org/chapters/embeddings.html#fig-pca-rotation) shows a pattern reminiscent of the horseshoe effect. We should mention this. See - [PCA ordination notes](https://ordination.okstate.edu/PCA.htm) - [What is the "horseshoe effect" and/or the "arch effect" in PCA / correspondence analysis?](https://stats.stackexchange.com/questions/158552/what-is-the-horseshoe-effect-and-or-the-arch-effect-in-pca-correspondence) - [Noise Reduction By Eigenvector Ordinations](https://doi.org/10.2307/1940105) - [Detecting the Guttman effect with the help of ordinal correspondence analysis in synchrotron X-ray diffraction data analysis](https://doi.org/10.1080/02664763.2020.1810644) - [Horseshoes in multidimensional scaling and local kernel methods](https://doi.org/10.1214/08-AOAS165)
The PCA plot of the barley data shows a pattern reminiscent of the horseshoe effect. We should mention this. See