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From |
"Nick Cox" <n.j.cox@durham.ac.uk> |

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

Subject |
RE: st: RE: Error Bars on Histogram or Bar Plots |

Date |
Mon, 2 Jun 2003 14:04:30 +0100 |

Buzz Burhans > > > 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) Let me try to divide this up into a variety of general and specific issues. As usual, discussion can range reasonably from the Stata-linked (how do you do X in Stata?) to the Stata-free (is X a good idea?). 1. Stata should support error bars on graphs ============================================ I agree with this. What's more, I don't think it's at all controversial. The only issue that is how it is supported. It so happens that Stata's been a bit laggard on producing wrapper commands for plots showing error bars. I doubt there's any policy there. However, having implemented various high- level commands in one way does inhibit twisting them in a different direction. -graph bar- for example is basically built around a -collapse- of the data. Building something else major on top of that, such as machinery for adding confidence intervals generally, would be, I guess, rather tricky. 2. Specific problems experienced by Buzz ======================================== Buzz: could you revisit this with concrete examples of what you typed and what is wrong or unsatisfactory about it? I may not be the only one to be unclear about precisely what syntax you are using. It is difficult, possibly even for the graphics experts at Stata, to know what you're missing which is there in Stata and what you're missing which is not. 3. How error bars should be shown ================================= I think there's a wider issue here about one kind of graph which I think Buzz has in mind. My impression is that it is very common in some (biological?) sciences but much less so in mainstream statistical literature. This graph shows magnitudes by thick bars with bases at 0 and errors by thin bars centred at the top of the bar, schematically multiples of _ | | XXX XXX XXX XXX XXX XXX XXX XXX ------- A couple of years ago at a London users' meeting (the same) Paul Seed commented that you couldn't draw such graphs easily in Stata (I don't recall if he wanted to draw them himself). As soon as he had explained what they were the reaction from the normally fairly polite and fairly articulate audience was something like "No, no." "Yukk." "Horrible!" My own rationale for disliking these graphs strongly is this: 1. Their visual signal conflates two distinct issues, estimate relative to zero and estimate relative to error bar. 2. Suppressing the bottom part of the error bar obliges the concerned user to guess mentally where it ends. That's very poor. 3. Such graphs take a lot of ink to show relatively little information. Something like a dot chart seems to me far preferable. Cleveland's work is relevant here. Of course, there will be other views, and there are other issues here, not least the anthropological one of what is expected or instructed by editors and referees in particular fields. Nick n.j.cox@durham.ac.uk * * 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/

**Follow-Ups**:**RE: st: RE: Error Bars on Histogram or Bar Plots***From:*Buzz Burhans <wsb2@cornell.edu>

**References**:**Re: st: RE: Error Bars on Histogram or Bar Plots***From:*Buzz Burhans <wsb2@cornell.edu>

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