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Re: st: RE: Error Bars on Histogram or Bar Plots


From   Buzz Burhans <wsb2@cornell.edu>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: RE: Error Bars on Histogram or Bar Plots
Date   Mon, 02 Jun 2003 06:40:55 -0400

> How do you put error bars on histograms or bar plots? I am
> using Stata 8 in Windows.

I don't think either is supported as a standard option.
I believe these were the posts that generated a great deal of consideration over the last week regarding sliding bins for histograms, histograms vs bar graphs, and the role of visual data exploration.

Having just spent a fair bit of time plotting some data for publication this weekend, I'd like to return to the original question regarding error bars. I have been using Stata 8, and I like the graphics. It seems though, that there is something fundamentally wrong with a statistics graphing program that doesn't facilitate the use of error bars on plots, whether connected line plots or bar graphs. Error or uncertainty is a major element of what we often wish or need to represent in a graphical representation of data. Stata makes one cob up solutions for this when it should be facilitating them. Yes, one can get it done, but not always easily or correctly for that matter. For instance,overlaying a range plot works, but I don't think one should have to have a marker on both ends of the "error bar", and I don't think that if the plotting symbols of the data are open symbols, the error bar should more than touch the edge...in fact there are journals that specifiy that there should not be markers in markers.

One of the best implementations of error bars in Stata has been Paul Seed's XTGRAPH; it is a nice implementation for a number of reasons: 1)It is built in to the process of plotting the data; 2) it allows a great deal of flexibility in the selection of the type of error one wants to represent; 3) It facilitates picking up and plotting the error terms from a previously run model 4) it facilitates visual issues by allowing half error bars or offsetting the plotted points.

It has some challanges for those of us who want to use it without hacking the code...it is not easy to control the symbol types, and the error bars come inside open symbols with their marker ends, but it is a really nice tool.

In any case, it is a nice example of a set of features that I think Stata should be more concerned about implementing. The issue of error terms seems fundamentally a part of what statisitics is about, and to have a powerful graphics package that essentially ignores the need to represent that element ( I recognize there is some implemention made vis a vis the CI plots). In the journals I read, error bars are commonly used, and it seems to me implementation of adding them to graphs easily and flexibly is an important feature that Stata Corp. should address. It should be as easy to add error bars to bar graphs, as the original post requested, and to other graph types as well (twoway connect for example)

Buzz Burhans

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