Introduction to Stata 8 - (GRIPS

twoway rspike
Cross-sectional study
// c:\dokumenter\...\gph.length_bias.do
clear
set obs 20
gen x=_n
gen y1=x
gen y2=y1+2
replace y2=y1+8 if mod(x,2)==0
set scheme lean2
twoway (rspike y1 y2 x , horizontal blwidth(*1.5)) ///
, ///
yscale(off) ylabel(, nogrid) ytitle("") ///
xlabel(none) xtitle("Cross-sectional study") ///
xline(14.5) ///
xsize(3.7) ysize(2.3) scale(1.4)
The purpose of this graph is to illustrate length bias: a cross-sectional (prevalence) study may
mislead you. Cases with short duration (due to successful treatment or high case fatality) are
underrepresented in a cross-sectional sample.
rspike is in the twoway r* family: range plots, like rcap shown before; this time it is
horizontal.
In range plots and droplines (next page) the lines technically are bar outlines, and options are
blcolor(), blpattern() etc.; hence the blwidth(*1.5) to make the spikes wider
than the default.
It is easy to create one or more reference lines; use xline() and yline().
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