We often see the terminology "confidence ellipse". This is not correct: in dimension 2, an ellipse is a curve, and the confidence region is the interior of this curve, not the curve. Similarly, in dimension 3, an ellipsoid is a surface, not a volume. And I don't know any name for the interior of an ellipsoid (though this is a ball for an appropriate metric).

What is the correct name, in arbitrary dimension ?

  • an ellipsoidal confidence region ?
  • an elliptic confidence region ?
  • an elliptical confidence region ?
  • another name ?

In French, my mother tongue, I would hesitate between "ellipsoïdal" and "elliptique". I don't know what is the French for "elliptical".

  • 2
    $\begingroup$ There are 400 thousand google hits for "area of an ellipse" which is strictly speaking a meaningless phrase if ellipse is defined as a curve (area of a curve is always zero). Personally, I am happy to tolerate this slight sloppiness. $\endgroup$
    – amoeba
    Aug 7, 2017 at 10:01

1 Answer 1


In common English use we regularly find people using "circle" to refer to the circular disc, not just its boundary, usually without confusion of what was intended. I agree that "ellipsoid" refers to the boundary while that is simply the bounds for the confidence region but I would have little hesitation in saying an ellipsoidal confidence region in spite of it being a bit of a fudging of the terminology.

If you wanted to be strict you could say something like a confidence region consisting of the interior of an ellipsoid but that seems slightly awkward; the less precise term would be unlikely to be misunderstood in most cases.

In some situations I might say something like "ellipsoidal ball"; while not all that common, it has sometimes been used -- see for example its use on page 652 here (near the middle of the page) -- and would likely be understood.

I'm unaware of a widely accepted single-word term for the interior; while one probably exists, enough people may be unfamiliar with it that it might not be the ideal choice anyway.


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