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Re: st: Graphing Quadratic Interactions


From   "Susanna Khavul" <skhavul@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Graphing Quadratic Interactions
Date   Tue, 16 Dec 2008 17:00:30 -0600

Thanks to everyone for the informative answers.  I appreciate the
insights. Will try.


On Tue, Dec 16, 2008 at 9:17 AM, Nick Cox <n.j.cox@durham.ac.uk> wrote:
> No, we can't erase that.
>
> Ronan has a good point, which I doubt is controversial anywhere, that
> just because you have 3 variables doesn't meet that you are best off
> with 3-D graphics.
>
> In particular if you have 3-D scatter, meaning a point cloud or swarm,
> 3-D graphics are not necessarily going to help you much. If you have 3
> categorical variables, the same is likely to apply.
>
> But plenty of us have data that define surfaces, often quite smooth
> ones, and for that use any 2-D projections or summaries don't capture
> the whole thing well. And if your data don't define a surface, your
> model predictions often will.
>
> Nick
> n.j.cox@durham.ac.uk
>
> Martin Weiss
>
>
> " This is not true, and all of the good high-dimensional charts I can
> think
> of use only two dimensions to display the data."
>
> So we can erase this perennial grievance ("When will Stata be able to
> draw
> 3-D graphs"?) from future "wishes & grumbles"...
>
> Ronan Conroy
>
> On 16 Noll 2008, at 12:42, Nick Cox wrote:
>
>> It wasn't very clear what she wants.  My guess was that she wants a
>> plot
>> of predicted response in terms of X4 and X5, adjusting for X1 ...
>> X3. As
>> mentioned on this list very frequently, Stata isn't serious at 3-D
>> graphics that might meet this need, whether contour plots or
>> perspective
>> views.
>
> What are often called 3D graphics are a small, and very poor, subset
> of the ways in which higher dimensional data can be displayed on a
> flat surface.
>
> Their immediate drawback is that of point of view. They represent a
> two-dimensional view of a three-dimensional object. Thus, the point of
> view must be carefully chosen if it is not to result in some
> interesting features being obscured, and there is no guarantee that
> any single view will show you all the features of your data.
>
> However, it is possible to display data in many dimensions on a flat
> surface, and we do this all the time. Use of -by- and -over- to
> display different groups in the same chart increases the number of
> dimensions while keeping all the data visible at once.
>
> . sysuse auto
> . stripplot mpg, by(foreign) over(rep78) bar
>
> This shows domestic and foreign cars as two graphs within the chart,
> allowing us to see a third dimension easily.
>
> You can, with a little ingenuity, increase the number of dimensions by
> using what Tufte calls 'small multiples' - a tiled arrangement of
> smaller graphs which follows a logical order.
>
> My own contribution to the field was making a chart that displayed
> age, sex, smoking, cholesterol, blood pressure and cardiovascular risk
> on a flat surface. The resulting chart is also technically a table,
> since it displays risk numerically as well as using a traffic-light
> colour system. You can see the thing in various places on the net,
> including Chance News
>
> http://www.dartmouth.edu/~chance/chance_news/recent_news/chance_news_13.
> 03.h
> tml#item10
>
> I mention this because people assume that you need "3D" to show more
> than two dimensions. This is not true, and all of the good high-
> dimensional charts I can think of use only two dimensions to display
> the data.
>
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