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From |
Subramanian Swaminathan <ssubra@yahoo.com> |

To |
statalist@hsphsun2.harvard.edu, nicola.baldini2@unibo.it |

Subject |
Re: st: how to use ztnb? |

Date |
Tue, 26 Feb 2008 01:41:09 -0800 (PST) |

dear Dr Nicola, I understand that the dispersion parameter 'k' (=mean*mean/(variance-mean)) of the negative binomial = 1/alpha, when dispersion is constant with no depedent variable (note: alpha is the coefficient of the estimated mean relating dispersion (variance to mean ratio) with mean in stata). But I want to model 'k' as a function of the estimated mean 'm' (i.e. k= k0+k1*m^beta), where m is the mean corresponding to the age-class given by a0+a1*age-class. I would like to test whether k1=0 against the alternatives k1<0 or k1>0. My aim is to test whether the dispersion 'k' declines with increasing age-class. I would like to know if I can model the relationship of 'k' with 'm' using the 'ztnb' option in stata. If possible, I request the stata group to help me out. with regards Subramanian Swaminathan Vector Control Research Centre (Indian Council of Medical Research) Indira Nagar Pondicherry - 605 006 INDIA ----- Original Message ----- From: <nicola.baldini2@unibo.it> To: <statalist@hsphsun2.harvard.edu> Cc: <ssubra@yahoo.com> Sent: Friday, February 22, 2008 11:27 PM Subject: Re: st: how to use ztnb? > Which is the question, here? Your knowledge of -ztnb- seems to exceed > mine. Your code > xi:ztnb mf-counts i.age [fweight = freq] if mf-counts>0, dispersion(mean) > let you estimate a zero truncated NB mobel where the dispersion (the > parameter called dispersion in Stata) is a nonlinear (exponential) > function of the mean. As a default, it includes a test about the > hypothesis that the dispersion is a nonlinear (exponential) function of > the mean vs. the dispersion is a constant (if the test is insignificant, > then the dispersion is a constant). > Nicola > > At 02.33 19/02/2008 -0500, Subramanian Swaminathan wrote: >>Dear stata listers, >> >>I am trying to fit a zero-truncated negative binomial >>model to my count >>data. The data variables are mf-counts, age, >>frequency. The model that I >>would like to fit is of the form: >>xi:ztnb mf-counts i.age [fweight = freq] if >>mf-counts>0, dispersion(mean) >> >>Kindly note that my dispersion parameter is not a >>constant but is a >>non-linear function as given below: >> >>k = k0+beta*mean**gamma (0<gamma < infinity) >> >>where, >>'k' is the dispersion parameter 'k' ( in stata, k=log >>(1/alpha) of the >>negative bonomial model >> >>beta & gamma = coefficients to be estimated >> >>In this model I want to test the hypothesis that >>beta=0 against beta not >>equal to 0. >> >>could anyone help me out, how the stata code should be >>written to estimate >>the parameters of the model? >> >>kindly note that i am new stata. > > ____________________________________________________________________________________ Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it now. http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ * * 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/

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