On Sat, 27 Mar 2021, Allin Cottrell wrote:
On Sat, 27 Mar 2021, Periklis Gogas wrote:
What is the actual algorithm that Gretl uses to estimate the Hurst
exponent?
Is it the Anis-Lloyd corrected R/S or the DFA?
You can see exactly what gretl is doing in
https://sourceforge.net/p/gretl/git/ci/master/tree/plugin/fractals.c
See hurst_calc() in particular.
If you find C code hard to read, here's a near-literal Hansl translation:
<hansl>
set verbose off
function scalar get_depth(scalar T)
return floor(log2(T) - 2)
end function
function scalar cum_range (const matrix x, scalar xbar)
c = cum(x - xbar)
return maxc(c) - minc(c)
end function
function matrix hurst_calc (const matrix x, scalar n, scalar depth)
matrix Z = mshape(NA, depth, 3)
m = n
loop i = 1 .. depth
RS = 0
nsub = floor(n/m)
loop j = 1 .. nsub
ini = 1 + (j-1)*m
fin = ini + m - 1
scalar xbar = meanc(x[ini:fin])
scalar r = cum_range(x[ini:fin], xbar)
s = sdc(x[ini:fin], 0)
RS += r / s
endloop
RS /= nsub
Z[i,1] = RS
Z[i,2] = log2(m)
Z[i,3] = log2(RS)
m = floor(m/2)
endloop
return Z
end function
### test with simulated data
nulldata 300
set seed 123
series x = normal()
depth = get_depth($nobs)
Z = hurst_calc({x}, $nobs, depth)
print Z
gnuplot 3 2 --matrix=Z --output=display
V = {}
b = mols(Z[,3], 1 ~ Z[,2], null, &V)
cs = b[2] ~ sqrt(diag(V)[2])
modprint cs "Hurst"
# compare with native
hurst x --plot=display
</hansl>
-------------------------------------------------------
Riccardo (Jack) Lucchetti
Dipartimento di Scienze Economiche e Sociali (DiSES)
Università Politecnica delle Marche
(formerly known as Università di Ancona)
r.lucche...@univpm.it
http://www2.econ.univpm.it/servizi/hpp/lucchetti
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