Dear Regina, I find the mention of 2 samples very confusing (this is also true for the Wikipedia site). This is especially problematic, because 2 samples will generate 2 different standard deviations, while there is only one sigma if you go with a population.
A far better description is provided on the Mathworks site. http://www.mathworks.com/access/helpdesk/help/toolbox/stats/index.html?/access/helpdesk/help/toolbox/stats/ztest.html&http://www.google.ro/url?sa=t&source=web&ct=res&cd=3&url=http%3A%2F%2Fwww.mathworks.com%2Faccess%2Fhelpdesk%2Fhelp%2Ftoolbox%2Fstats%2Fztest.html&ei=qt-TSvKEKoPe-QbviqyxBg&rct=j&q=ztest&usg=AFQjCNEynERcFmdEW0pD-B-nZSBuh7zYPw "The p-value is the probability, under the null hypothesis, of observing a value as extreme or more extreme of the test statistic <z> where <Xm> is the sample mean, μ = m is the hypothesized population mean, σ is the population standard deviation, and n is the sample size." This is the classical description of the z-statistic. As I mentioned on a number of occasions, ztest is not implemented directly in R (as it should be avoided in any serious statistic - and has no place there). I would therefore stick with the one sample definition, and adapt only the text to correspond to what actually the function computes. "The p-value is the probability, under the null hypothesis, of observing a value as extreme or more extreme of the z-statistic" or shortened: "calculates the probability of observing a value as extreme or more extreme of the z-statistic" and (possibly) correcting for the wrong implementation: "calculates the probability of observing a value as large or larger for the z-statistic" and, if space is really such a huge concern, then compacting "as large or larger" will result in: "calculates the probability of observing a value larger than the z-statistic" Sincerely, Leonard Mada -------- Original-Nachricht -------- > Datum: Tue, 25 Aug 2009 14:23:33 +0200 > Von: Regina Henschel <[email protected]> > An: [email protected] > Betreff: Re: [sc-dev] Re: [Issue 90759] ZTEST not same as Excel > Hi Leonard, > > Leonard Mada schrieb: > > Dear Calc team, > > > > the following wording is ambiguous and it may be wrong altogether: > > > >> function description: > >> calculates the probability of a *sample* mean greater than the mean of > >> the given *sample*. > > [EMPHASIS ADDED] > > A long phrase for the application help would be: > > For a given random sample of size n, drawn from a normally distributed > population with a known mean µ and standard deviation sigma, ZTEST > calculates the probability that another sample of the same size would > have a mean greater than the mean m of the given sample. > > ZTEST calculates 1-NORMSDIST(z) where z = (m-µ)/(sigma/sqrt(n)). > > > The function ZTEST is not a Z-test, but you can calculate a value by > ZTEST, which you can use to perform a Z-test. > > Do you know a better phrase, that *does not exceed two lines* for the > function wizard? We can explain the function in detail on the Wiki, > where you already find the formula and nice diagramms. The problem is, > to get a very short description, without given such useless phrases like > for TTest, where you find "Calculates the T test" in the function wizard > and "Returns the probability associated with a Student's t-Test." in the > application help. > > kind regards > Regina > > > > --------------------------------------------------------------------- > To unsubscribe, e-mail: [email protected] > For additional commands, e-mail: [email protected] -- Jetzt kostenlos herunterladen: Internet Explorer 8 und Mozilla Firefox 3 - sicherer, schneller und einfacher! http://portal.gmx.net/de/go/atbrowser --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
