Oct 072012
 
Matrix Graph in Stata

Do you like this graph? I don’t think it is particularly attractive, and that is after spending hours and hours creating it. What I really wanted was a matrix-like representation of 18 simulations I ran. More specifically, I simulated the sampling distribution of a statistic under six different conditions for three different sample sizes. Doing the simulations was a breeze, courtesy of Stata’s simulate command, which created 18 corresponding data sets. Graphing them with kdensity also poses no problem, but combining these graphs did, because I could find no canned command that produces what I wanted: a table-like arrangement, with labels for the columns (i.e. sample sizes) and rows (experimental conditions). What I could do was set up / label a variable with 18 categories (one for each data set) and use the ,by() option to create a trellis plot. But that would waste a lot of ink/space by replicating redundant information. At the end of the day, I created a nine graphs that were completely empty save for the text that I wanted as row/column labels, which I then combined into two separate figures, that were then combined (using a distorted aspect ratio) with my 18 separate plots. That boils down to a lot of dumb code. E.g., this creates the labels for the six conditions. Note the fxsize option that makes the combine graph narrow, and the necessity to create an empty scatter plot.

capture drop x
capture drop y
capture set obs 5
gen x= .
gen y= .

local allgraphs = “”

forvalues c = 1/6 {
graph twoway scatter x y, xtitle(“”) ytitle(“”) xscale(off) yscale(off) subtitle(“(`c’)”,position(0) nobox) graphregion(margin(zero)) plotregion(style(none))
local allgraphs “`allgraphs’ condition`c'”
graph rename condition`c’ , replace
}

graph combine `allgraphs’ , cols(1) colfirst imargin(0 0 0 0) fxsize(10) b1title(” “)

The column labels were created by similar code. Finally, I combined my 18 graphs (there names are in the local macro) and combined the results with the label graphs.

graph combine `graphs' ,colfirst cols(3) ycommon xcommon imargin(3 3 3 3) b1title("\$ B_w \$") l1title("Density")
graph rename simulations, replace
graph combine sizelabels.gph condlabels.gph simulations, imargin(0 0 0 0) cols(2) holes(1)

Can anyone of you think of a more elegant way to achieve this result?

Matrix Graph in Stata

 

Jan 232010
 

I use emacs/LaTeXfor all my textprocessing needs, and for the last four or five years, I have created all my slides with Till Tantaus excellent “beamer” class. At the moment, I’m teaching a 2nd year stats course (imagine doing this with PowerPoint – the horror! the horror!), so I sometimes use graphs from the assigned text like this one from Long&Freese that illustrates the latent variable/threshold interpretation of the binary logit model. The message should be fairly clear: y^{*} depends on x andfollows a standard logistic distribution around its conditional mean.

Click to download the animated PDF. Requires javascript, so view in Acrobat reader.

But the fact that the bell-curve lies flat in the x-y^{*} plane confused my students no end. So I wasted half a day on creating a nice 3d-plot for them. After trying several options, I settled on pgfplots.sty, which builds on tikz/pgf, the comprehensive, portable graphics package designed by Tantau (here’s a gallery with most amazing examples of what you can do with this little gem). Plotting data and functions with pgfplots in 2d or 3d is a snap, so that was not too hard. Eventually.

Finally, in a desperate attempt to drive the message home, I enlisted the help of animate.sty, yet another amazing package that creates a javascript-based inline animation from my LaTeX source (requires Acrobat reader). So the bell-curves pop out of the plane, in slow motion. Did it help the students to see the light? I have no idea. Here is the source.