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From |
Richard Williams <richardwilliams.ndu@gmail.com> |

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

Subject |
Re: st: Interpretation of coefficients for predictor with linear and quadratic terms in negative binomial regression |

Date |
Mon, 21 Oct 2013 10:24:08 -0500 |

At 11:10 PM 10/20/2013, Rachael Wills wrote:

Dear all,I am trying to quantify the effect of the occupancy (%) of childcare centres on the tendency of children to take extended departuresfrom the centre.I am using -nbreg- in Stata MP 12.1 to run a negative binomialregression where the outcome is the number of children at a centretaking an extended departure during the year (variable is called'exits') with an offset term containing the number of childrenattending the centre at all during the year (variable is called'children'). A scatterplot of 'exits' as a percentage of 'children'against occupancy (%) suggests that a term in occupancy squared mayalso be necessary, and indeed both linear and quadratic terms aresignificant at the 95% level in the model:gen occ2 = occ^2 nbreg exits occ occ2, exposure(children)My question then, is how I can interpret the dual coefficients forthe occupancy terms. Is it best to use the coefficients, or can asimpler interpretation be made using the -irr- option? I would liketo be able to provide a statement such as 'For every 1% increase inoccupancy there is a X decrease in the exit rate'. However, I'm noteven sure if such a simple statement is possible when there are bothlinear and quadratic terms involved.

Personally I would do nbreg exits occ c.occ#c.occ, exposure(children) Then I would use commands like margins or the user written mcp. See http://www3.nd.edu/~rwilliam/xsoc73994/Margins01.pptx http://www3.nd.edu/~rwilliam/xsoc73994/Margins03.pdf You may also wish to see Vince Wiggins' post at http://www.stata.com/statalist/archive/2013-01/msg00293.html

------------------------------------------- Richard Williams, Notre Dame Dept of Sociology OFFICE: (574)631-6668, (574)631-6463 HOME: (574)289-5227 EMAIL: Richard.A.Williams.5@ND.Edu WWW: http://www.nd.edu/~rwilliam * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/faqs/resources/statalist-faq/ * http://www.ats.ucla.edu/stat/stata/

**References**:**st: Interpretation of coefficients for predictor with linear and quadratic terms in negative binomial regression***From:*Rachael Wills <rwills@goodstart.org.au>

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