Talk:Semivariance

Can you please tell my what the POV issue is? It will never be fixed if the tag is left unexplained; & if the issue is the controversy section, well, then its no longer POV....Bridesmill 16:25, 5 July 2006 (UTC)

deleted nonsense
I just deleted this statement:


 * In mathematical statistics, sets of n measured values give df=n-1 degrees of freedom whereas

Quite aside from the fact that


 * df=n-1

should have been typeset as


 * df = n &minus; 1

the statement itself is idiotic nonsense. Michael Hardy 21:38, 17 December 2006 (UTC)

Variogram, semivariogram and others
I first cite: ''The semivariance is calculated in the same manner as the variance but only those observations that fall below the mean are included in the calculation. It is sometimes described as a measure of downside risk in an investments context. For skewed distributions, the semivariance can provide additional information that a variance does not.'' This cannot be correct. I have removed it. --Bachmai (talk) 15:39, 23 February 2008 (UTC)

I have described the context between variance and "semivariance" in Bachmaier and Backes (2008) cited in the article. This context is not yet written down in this article. (Martin Bachmaier) --Bachmai (talk) 15:39, 23 February 2008 (UTC)

Someone wrote: ''The statement that only measured values below the mean are included in the semivariance makes no statistical sense (see Ref 4). Clark, in her Practical Geostatistics, which can be downloaded from her website, proposed that the factor 2 be moved for mathematical convenience and berates those who refer to variograms rather than semi-variograms.'' The first part is o.k. And I think it's me who removed what you criticized. But as I wrote in my paper cited in the article, to which one reviewer said that it blaims professionals, variance and variogram are the only terms that should be used. The problem is that the variance formula which has the factor 1/2 and is the basis of the variance at given separation distance (unfortunately called semivariance) is less known. See the German empirische Varianz, where this alternative is described. My paper confers all standpoints. It needs to be known that the semivariance is nothing else than a variance restricted to certain pairs with a distance of h (that has nothing to do with negative deviations or similar). --Bachmai (talk)

Clark can be right when she consequently calls the squared sum the variance of the differece of z values. However, the problem is that people only say that it is the semivariance without saying of what. It's me who has added it now. People say that gamma is the semivariance of yield values for example. However, gamma is the semivariance of yield value differences.--Bachmai (talk) 19:51, 23 February 2008 (UTC)

Therefore:
 * gamma = 0.5 sum... variance of z at given h = semivariance of z differences
 * 2gamma = sum... (according to Clark) = twice the variance of z at given h = variance of z differences (which is usually not said)

--Bachmai (talk) 19:51, 23 February 2008 (UTC)

Before I wrote here, gamma = sum... was defined. (gamma instead of 2gamma!!!) This is nowhere (perhaps in Clark, which is one of a few books which I have not looked at). These people normally say 2gamma = sum... --Bachmai (talk) 19:51, 23 February 2008 (UTC)

The main problem is that "semivariogram" is endemic in the general scientific literature, and not just in one field of science. I can mention two additional books: The statement "Bachmaier and Backes (2008), who discussed this confusion, have shown that..." should at least be toned down to something like "say they think that". See also variogram. Melcombe (talk) 15:52, 17 April 2008 (UTC)
 * Stein ML (1999) Interpolation of Spatial Data. Spinger.
 * Schabenberger O., Gotway CA. (2005) Statistical Methods for Spatial Data Analysis, Chapman&Hall/CRC.

Comments for improved clarity
The header "The neutrality of this article is disputed..." appears to be resolved. I don't see anything remaining in the article that appears non-neutral. Not knowing anything previously about the topic, I'm not going to make the final judgement call to actually remove it...

A simple illustrative example with, say, 10 data points would help to make it a little more readily accessible to those of us who haven't looked at this previously.

Is there a reason that "Semideviation" doesn't link here. Is Semideviation the square root of semivariance?

Is this the same concept that Markowitz explored originally in his 1950s work on capital asset pricing models? I have heard he originally focused on semi-deviation to capture risk, but eventually settled on standard deviation in his now famous CAPM work. I got here because I was curious about this, but I'm not sure whether this is the same concept.

At the top is says "the empirical semivariance is...". Does this imply that a non-empirical definition exists, for example, given a continuous distribution P(x) is there a notion of the semivariance of this analytic distribution? If so, the dof controversy would be a bit more concrete also, since we could talk about whether the empircal semivariance is an unbiased estimator of the underlying semivariance. Otherwise, it sounds like vague handwaving. —Preceding unsigned comment added by Ldc (talk • contribs) 18:41, 19 May 2010 (UTC)

The variables in the equation are not described, making it difficult for those without a statistical background to understand. — Preceding unsigned comment added by JWS (talk • contribs) 21:17, 6 July 2012 (UTC)

Other definition of semivariance
Someone wrote above six years ago


 * I first cite: The semivariance is calculated in the same manner as the variance but only those observations that fall below the mean are included in the calculation. It is sometimes described as a measure of downside risk in an investments context. For skewed distributions, the semivariance can provide additional information that a variance does not. This cannot be correct. I have removed it.

Actually it is exactly right for a different usage of the term. I'll put in something to disambiguate this when I get a chance. Loraof (talk) 17:21, 27 October 2014 (UTC)

Intro
Someone should add an intro explaining what the semivariance is useful for (e.g. with an example). Ben Finn (talk) 11:14, 27 August 2015 (UTC)

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