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From |
jpitblado@stata.com (Jeff Pitblado, Stata Corp.) |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: gr tw (scatter if) (scatter if), by() |

Date |
Mon, 05 May 2003 13:15:47 -0500 |

David Airey <david.airey@vanderbilt.edu> asks about using the -by()- option, with multiple overlays and the -if- option: > .scatter lens_average eye_nolens if parity <= 9, by(parity) > > gives a 3 x 3 graph of nine plots (parity 1, 2 to 9) > > .tw lfit lens_a eye_n if parity <= 9, by(parity) > > gives a 3 x 3 graph of nine plot > > .tw (scatter lens_average eye_nolens if parity <= 9) /// > (lfit lens_average eye_nolens if parity <= 9), by(parity) > > gives the message: > > . tw (scatter lens_average eye_nolens if parity <= 9) /// > > (lfit lens_average eye_nolens if parity <= 9), by(parity) > no observations > > but completes a graph (3 x 4) leaving three holes in positions 10-12. > > Is this behavior intended? > > Given the first two graphs, I would have thought the by() option would > travel through the overlay of the plots properly. It doesn't, at least > not the way I intended. > > Maybe I should have simply done, > > preserve > drop if parity > 9 > tw (scatter lens_average eye_nolens) /// > (lfit lens_average eye_nolens), by(parity) > restore So David would like to have each scatter plot in: . tw scatter lens_average eye_nolens if parity <= 9, by(parity) overlayed with the regression line(s) from (respectively): . tw lfit lens_average eye_nolens if parity <= 9, by(parity) And David tried: . tw (scatter lens_average eye_nolens if parity <= 9) /// (lfit lens_average eye_nolens if parity <= 9), by(parity) but didn't get what he wanted. In the first two graph commands the if-condition "if parity <= 9" applied to the graph "globally", and thus restricted the data available to the -by()- option. In the third command, the "if parity <= 9" conditions are now "local", thus the -by()- option is not restricted to only work with the data that allowed by the (albeit common) if conditions. The solution for David's problem is to specify the -if- restriction globally: . tw (scatter lens_average eye_nolens) /// (lfit lens_average eye_nolens) /// if parity <= 9, by(parity) --Jeff jpitblado@stata.com * * 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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