A regression threat, also known as a "regression artifact" or "regression to the mean" is a statistical phenomenon that occurs whenever you have a nonrandom sample from a population and two measures that are imperfectly correlated. The figure shows the regression to the mean phenomenon. The top part of the figure shows the pretest distribution for a population. Pretest scores are "normally" distributed, the frequency distribution looks like a "bell-shaped" curve. Assume that the sample for your study was selected exclusively from the low pretest scorers. You can see on the top part of the figure where their pretest mean is -- clearly, it is considerably below the population average. What would we predict the posttest to look like? First, let's assume that your program or treatment doesn't work at all (the "null" case). Our naive assumption would be that our sample would score just as badly on the posttest as they did on the pretest. But they don't! The bottom of the figure shows where the sample's posttest mean would have been without regression and where it actually is. In actuality, the sample's posttest mean wound up closer to the posttest population mean than their pretest mean was to the pretest population mean. In other words, the sample's mean appears to regress toward the mean of the population from pretest to posttest.


Lepo pretpostavih da je reč o kliničkoj studiji u vezi sa epilepsijom, te statističkoj analizi rezultata iste. Dakle, npr. u tzv. regresionim analizama rezultata, kakva je Cox-ova, često se koristi termin "regression to the mean" ili "regression artifact/regression threat". Reč je o regresiji po medijani (srednjoj vrednosti nekog broja).


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Note added at 29 mins (2008-05-23 15:22:00 GMT)
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dr Milan Merkle: Medijana: klasicni i noviji rezultati.

Medijana kao mera srednje vrednosti, u poredjenju sa matematickim ocekivanjem ima prednost u robusnosti i bezuslovnoj egzistenciji, a nedostatak u nelinearnosti i nejedinstvenosti. U klasicnim tekstovima iz teorije Verovatnoce malo paznje je posveceno medijani, ali s druge strane koncepti kao sto su nepristrasnost po medijani, regresija po medijani, uslovna medijana i sl. nalaze veliku primenu u alternativnim resenjima tradicionalnih problema matematicke statistike. Izlaganje ce biti posveceno nekim rezultatima koji se odnose na osobine medijane, kao i nekim primenama teorije bazirane na medijani umesto na matematickom ocekivanju.

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Note added at 1 day22 hrs (2008-05-25 13:21:14 GMT) Post-grading
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Najlepše zahvaljujem! :) Bitno je da je bilo od pomoći. :)