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This question already has an answer here:

I need to understand how to read kernel density graphs. How do you come up with the values in y-axis?enter image description here

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marked as duplicate by Andy W, whuber Apr 9 '13 at 17:38

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The Y-axis in a kernal density estimate of the PDF of a random variable is interpreted as a probability differential, like in calculus. It's a unitless measurement but any mesaurable area under the curve can be interpreted as a probability estimate.

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    $\begingroup$ so given yaxis being 0.07, what does this mean in plain English. I understand histogram frequency charts, but this seems complicated to me. Can you clarify this little bit more? $\endgroup$ – user1471980 Apr 9 '13 at 17:18
  • $\begingroup$ The linked CV question from Huber should address your technical knowledge gap here. The literal interpretation of the Y-axis is a dimensionless quantity, like an infinitesimal grain of probability sand. An interpretation of a smoothed density estimate that's appropriate for applied audiences and scholarly journals is to calculate estimated "probability area" in pre-specified ranges. If the X-axis here were times to recurrence of cancer in months following therapy, you might be interested in risk of 1 year recurrence which you could estimate by using the function integrate(...). $\endgroup$ – AdamO Apr 9 '13 at 21:26

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