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In the following plot, I am trying to find the correlation between two different measures:

enter image description here

I'm confused about the result of the p-value which is below the cut-off p-value of 0.05. Looking at the plot, I was expecting there will be no correlation between the two measures.

My question is it really true there is a significant correlation between the two measures? and should I say that we reject the null hypothesis and accept the alternative instead?

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    $\begingroup$ What is the blue line along the top of the plot? How many observations do you have as I suspect there is much over-printing here which makes it difficult to see what is going on? $\endgroup$
    – mdewey
    Aug 4, 2019 at 16:45
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    $\begingroup$ Agree with @mdewey. Perhaps the few points at bottom center along with a slight concentration of points at upper right are enough to give a significantly positive Spearman correlation. Also many overplotted points at upper left might be slightly lower than points at upper right. Maybe start with temporary deletion of points below .5 on y axis, trying for a scale with less overplotting. // I'm wondering if there any useful association. $\endgroup$
    – BruceET
    Aug 4, 2019 at 17:13
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    $\begingroup$ If you consider Spearman's correlation appropriate to summarize the association between the variables and rejecting its null hypothesis is of relevance, then everything seems okay. $\endgroup$
    – Michael M
    Aug 4, 2019 at 18:05
  • $\begingroup$ @mdewey The blue line is a regression line (I accidentally added it to the ggscatter plot in R). The number of observations is 7290 so there is no overplotting. $\endgroup$
    – Adam Amin
    Aug 5, 2019 at 10:57

1 Answer 1

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Comment continued, with graphs using R.

set.seed(2019)
y = rbeta(1000, 10, .4)
x = runif(1000)
par(mfrow=c(1,2))
 plot(x,y,pch=20,ylim=0:1)
 plot(x,y^20,pch=20, ylim=0:1)
par(mfrow=c(1,1))

enter image description here

Data for both plots have the same Spearman correlation.

cor(x,y, meth="sp")
[1] 0.01978538
cor(x,y^20, meth="sp")
[1] 0.01978538
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  • $\begingroup$ Can I say in this case there is a significant correlation between measure x and measure y? $\endgroup$
    – Adam Amin
    Aug 5, 2019 at 11:03
  • $\begingroup$ I couldn't edit my last comment so I have to add a new comment. If there is a significant correlation, is this because of some data points, but not all points, that show a correlation between measure x and y? $\endgroup$
    – Adam Amin
    Aug 5, 2019 at 11:29

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