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To apply and visualize PCA in R often ggbiplot() is used. What is the meaning of this plot ? Why it is useful for PCA ?

We can consider this example: https://tgmstat.wordpress.com/2013/11/28/computing-and-visualizing-pca-in-r/

Figure

In particular, what is the meaning of arrows and circle?

amoeba
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Ef_Ci
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  • Thanks for comments. I have seen the posts. I have understood that arrows are linked to correlation: more correlated are two variables and more collinear are the arrows. It is true? And what is the precise relation? – Ef_Ci Sep 01 '16 at 23:55
  • And what about the white circle? Thanks for your consideration! – Ef_Ci Sep 01 '16 at 23:56
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    You may also want to read [Positioning the arrows on a PCA biplot](http://stats.stackexchange.com/q/141085/). – gung - Reinstate Monica Sep 02 '16 at 00:03
  • I think you will find the information you need in the linked thread. Please read it. If it isn't what you want / you still have a question afterwards, come back here & edit your question to state what you learned & what you still need to know. Then we can provide the information you need without just duplicating material elsewhere that already didn't help you. – gung - Reinstate Monica Sep 02 '16 at 00:04
  • Regarding the white circle, see "Visualizing a million" thread: "The white circle indicates the theoretical maximum extent of the arrows." – amoeba Sep 02 '16 at 09:20
  • I have read this comment in the old question but there is a problem: what is the length of arrows? I think is the projection of the original variables on the subspace generated by pc1 and pc2 but i'm not sure – Ef_Ci Sep 02 '16 at 13:23
  • The author of the ggbiplot package is the same user who posted the answer in the "Visualizing a million" question. Please see this old comment exchange: http://stats.stackexchange.com/questions/7860/visualizing-a-million-pca-edition#comment253864_7862 (last three comments). The details of how the circle+arrows are scaled are not very clear to me, and he never replied. – amoeba Sep 02 '16 at 23:51
  • Ok. If I understand more I will write here! – Ef_Ci Sep 03 '16 at 00:04

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