Consider the difference between r(rho)^2 and `r(rho)'^2 when r(rho) is
negative.
-display- understands the calculation when you use the first but if you
use the second then it sees (e.g.) -0.7^2 when you want it to act as it
would if it saw (-0.7)^2.
How Stata behaves in this circumstance is the subject of an FAQ:
FAQ . . . . . . . . . . . . . . The use of macros with the power
operator
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . W.
Gould
12/99 I coded `a'^2 and got the wrong sign. Why?
http://www.stata.com/support/faqs/lang/macpow.html
As the FAQ implies, this issue is not intrinsic to r() stuff, but
generic to the use of macros in expressions, which are always
substituted first, before any evaluation.
The lesson is to parenthesise aggressively.
Nick
[email protected]
Aleksander Rutkowski
Why the following two formulas - with and without quotes around r()
macros (see below) - produce very different results if the value stored
in the r() macro is negative? I attach a part of a Stata output
(Stata10SE, WindowsXP). Should not all macros, including the r() macros,
be quoted?
corr empl0 empl1
local t = r(rho) * sqrt((r(N)-2)/(1-r(rho)^2))
local t2 = `r(rho)' * sqrt((`r(N)'-2)/(1-`r(rho)'^2))
local p = 2*ttail(r(N)-2,abs(`t'))
local p2 = 2*ttail(`r(N)'-2,abs(`t2'))
di "r(rho): " r(rho) " r(rho)': " `r(rho)'
di "r(N): " r(N) " r(N)': " `r(N)'
di "t: " `t' " t2: " `t2' " p: " `p' " p2: " `p2'
di
Example of Stata output:
r(rho): -.58133648 r(rho)': -.58133648
r(N): 10 r(N)': 10
t: -2.0208211 t2: -1.4215178 p: .07795928 p2: .19295556
r(rho): .85050034 r(rho)': .85050034
r(N): 10 r(N)': 10
t: 4.5735645 t2: 4.5735645 p: .00181727 p2: .00181727
r(rho): .00749718 r(rho)': .00749718
r(N): 10 r(N)': 10
t: .02120583 t2: .02120583 p: .98360083 p2: .98360083
r(rho): -.7597187 r(rho)': -.7597187
r(N): 10 r(N)': 10
t: -3.3045795 t2: -1.7110323 p: .01078497 p2: .12543946
r(rho): .18673805 r(rho)': .18673805
r(N): 10 r(N)': 10
t: .53763206 t2: .53763206 p: .60545943 p2: .60545943
r(rho): .64313145 r(rho)': .64313145
r(N): 10 r(N)': 10
t: 2.3754968 t2: 2.3754968 p: .04485987 p2: .04485987
r(rho): .74453652 r(rho)': .74453652
r(N): 10 r(N)': 10
t: 3.1544711 t2: 3.1544711 p: .01350709 p2: .01350709
r(rho): .88701037 r(rho)': .88701037
r(N): 10 r(N)': 10
t: 5.4333446 t2: 5.4333446 p: .00062087 p2: .00062087
r(rho): -.2475888 r(rho)': -.2475888
r(N): 10 r(N)': 10
t: -.72279083 t2: -.67976198 p: .49039643 p2: .51583887
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