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From |
"Allan Reese (Cefas)" <[email protected]> |

To |
<[email protected]> |

Subject |
RE: st: plotting a regression function with time-dummies indicating structural breaks |

Date |
Tue, 10 May 2011 14:25:58 +0100 |

Back in March I had a similar problem to Oliver Jones and discussed off-list with Nick Cox (see below, reading from end). We let it drop, but it clearly confuses more users than just me that "graph twoway function" operates without the data matrix. Using names of variables in the function to be plotted produces unexpected results. Maybe the developers could look at the parsing and issue warning messages, and at the documentation to clarify that italic "x" is a literal and not (like "exp" and "range") a token to be substituted. The better approach, when the X axis is a data variable, is Maarten's suggestion of generating points on the line and -connect-ing them. Allan R Allan Reese Senior statistician, Cefas The Nothe, Weymouth DT4 8UB --- Oliver Jones asked (statalist 3 May) >> I wonder if there is an easy way to plot a regression of the form: >> >> y_t b_0 + b_1*TIME_t + Dummy_1*(b_2 + b_3*TIME_t) + b_4*Dummy_2 [where b_0 etc are scalars and Dummy_1 etc are variables] >> >> So far I tried: >> gen x TIME_t >> twoway /// >> (tsline y) /// >> (function y b_0 + b_1*x + Dummy_1*(b_2 + b_3*x) + b_4*Dummy_2, /// >> range(`first_year' `last_year')) & Maarten buis replied: > I tend to plot such graphs by first using -predict- or -adjust- to > get (adjusted) predictions and than use -twoway line-. It is just > too easy to make a typo in this type of -twoway function- calls. ------------------------------------------------------------------- ----- From: Allan Reese (Cefas) [mailto:[email protected]] Sent: 18 March 2011 13:50 To: Nick Cox Subject: RE: Bug in "twoway function" (v 11.1 used) It's a nice exercise for the little grey cells - years of training that f(x) is just a function led to assumption I should substitute the "x" variable in the function call. Expression parsing should allow the program to determine that f(x) with no x term should be constant. As it's not necessarily evaluating f() where the data has values and -price- is not a function, I would agree with your last point. Allan ---- From: Nick Cox [mailto:[email protected]] Sent: 18 March 2011 13:36 To: Allan Reese (Cefas); 'Stata Technical Support' Subject: RE: Bug in "twoway function" (v 11.1 used) A better answer, perhaps, is that Stata takes your RHS in [LHS ] RHS and evaluates it, after substituting for any x with a grid of points on [0,1]. If you rescale the range, it does so too. The surprise is that it doesn't actually throw you out if there is no reference to x. Nick [email protected] ---- From: Nick Cox Sent: 18 March 2011 13:16 To: 'Allan Reese (Cefas)'; Stata Technical Support Subject: RE: Bug in "twoway function" (v 11.1 used) I think the question is whether this is Stata's misfeature or yours. Perhaps -twoway function- should reject your syntax whenever it doesn't use -x-, which is explicitly expected, but it does the best it can to make sense of what you said. The effect appears to be equivalent to gen obs _n scatter price obs but scaled to the range of price. Evidently your syntax is not illegal, otherwise it would have been rejected, but it does not produce anything useful. I think this is on the level of if you ask a strange question, you may get a puzzled reply! Nick [email protected] ---- From: Allan Reese (Cefas) [mailto:[email protected]] Sent: 18 March 2011 11:11 To: Stata Technical Support Cc: Nick Cox Subject: Bug in "twoway function" (v 11.1 used) Documentation for -twoway function- states: You type -twoway function ysqrt(x)- It makes no difference whether y and x are variables in your data. Unfortunately it does. Using the auto data, compare . scatter weight price || function yx, range(price) n(20) . scatter weight price || function yprice, range(price) n(20) . scatter weight price || function yprice, range(price) Copy price as "x" and plot yx and it works, so what was meant appears to be that you -must- define the f(x) in terms of the dummy "x" and a variable called "x" in the dataset will not be used. But it is not obvious to me how price is used in the second and third examples. As there is no x, the function ought to be constant. It's also confusing to write that the "... in range play[s] no part unless option range(varname) is specified." The -in range- selects cases. Of course, only an idiot would not understand f(x)... Allan * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

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