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From |
"Herve STOLOWY" <stolowy@hec.fr> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
=?UTF-8?Q?R=C3=A9p.=20:=20Re:=20st:=20Interpretation=20of=20-mfx?==?UTF-8?Q?-?= |

Date |
Thu, 19 Jun 2008 07:34:55 +0200 |

Dear Maarten: I sincerely appreciate your detailed reply (including the other e-mail). It's very clear and helps me a lot. I have though a secondary question of interpretation. I use the auto file and make a logit regression followed by mfx. sysuse auto logit foreign mpg weight price mfx Here are the outputs: ------------------------------------------------------------------------------ foreign | Coef. Std. Err. z P>|z| [95% Conf. Interval] -------------+---------------------------------------------------------------- mpg | -.1210918 .0956825 -1.27 0.206 -.3086261 .0664424 weight | -.0068497 .0019993 -3.43 0.001 -.0107682 -.0029312 price | .0009264 .0003073 3.01 0.003 .000324 .0015287 _cons | 14.42237 5.413925 2.66 0.008 3.811276 25.03347 ------------------------------------------------------------------------------ Marginal effects after logit y = Pr(foreign) (predict) = .04198211 ------------------------------------------------------------------------------ variable | dy/dx Std. Err. z P>|z| [ 95% C.I. ] X ---------+-------------------------------------------------------------------- mpg | -.0048703 .00566 -0.86 0.390 -.01597 .006229 21.2973 weight | -.0002755 .00021 -1.33 0.184 -.000682 .000131 3019.46 price | .0000373 .00003 1.36 0.175 -.000017 .000091 6165.26 Here is my question: in the logit ouput, the coefficients on weight and price are significant. In the mfx output, the corresponding coefficients of dy/dx are NOT significant. How should I interpret this result? Best regards Herve *********************************************************** Professeur/Professor President of the French Accounting Association (AFC) HEC Paris Departement Comptabilite Controle de gestion / Dept of Accounting and Management Control 1, rue de la Liberation 78351 - Jouy-en-Josas France Tel: +33 1 39 67 94 42 - Fax: +33 1 39 67 70 86 mail: stolowy at hec dot fr web: http://www.hec.fr/stolowy >>> Maarten buis <maartenbuis@yahoo.co.uk> 12:18:36 pm 6/18/2008 >>> --- Herve STOLOWY <stolowy@hec.fr> wrote: > I run -mfx- after -logit- and have some difficulties to interpret > some of the elements displayed in the mfx output. <snip> > I understand the meaning of dy/dx, 95% CI and X. However, I don't > know how to read the z and associated p-value. What is the underlying > test? The standard error is the standard error of dy/dx. The z is the test statistic of the test that dy/dx equals zero, and the reported p-value is its p-value. > Is the Std. Err. an important element to display? In other > terms, if you want to present some of the elements of the output in a > paper, which ones would you chose? dy/dx? Std. Err? z? p(z)? CI? X? This is largely a matter of style. What you want is report the point estimate (dy/dx) and some measure of uncertainty, this could be either the standard error, the z, or the CI. It sounds a bit weird to report the z, but in fact it usually more convenient for the reader than reporting the standard error. Think about how you would interpret a table with coefficients and standard errors: you would look for each coefficient whether the coefficient is less than twice the standard error or not. If you report the z-value all they need to do is look at whether the reported z-value is less than 2 or not. It does not really matter which one of these three you choose because it is pretty easy to recover the other statics from any one of these (in combination with the point estimate). Some people would only report the p-value. The problem here is that recovering the other statistics from the p-value is much harder, as usually substantial rounding takes place. Even worse is only reporting whether or not a coefficient is significant or not (so-called "gazing at the stars"). Now it is completely impossible to recover any of the other statistics. Anyhow, the choice of what to report is usualy not yours to make, as most journals have pretty strict guidelines on what you should report. -- Maarten ----------------------------------------- Maarten L. Buis Department of Social Research Methodology Vrije Universiteit Amsterdam Boelelaan 1081 1081 HV Amsterdam The Netherlands visiting address: Buitenveldertselaan 3 (Metropolitan), room Z434 +31 20 5986715 http://home.fsw.vu.nl/m.buis/ ----------------------------------------- __________________________________________________________ Sent from Yahoo! Mail. A Smarter Email http://uk.docs.yahoo.com/nowyoucan.html * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/ * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**Re: Rép. : Re: st: Interpretation of -mfx-***From:*Maarten buis <maartenbuis@yahoo.co.uk>

**References**:**st: Interpretation of -mfx-***From:*"Herve STOLOWY" <stolowy@hec.fr>

**Re: st: Interpretation of -mfx-***From:*Maarten buis <maartenbuis@yahoo.co.uk>

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