function xsi = tail(empirical,analytical)
%function xsi = tail(empirical,analytical)
% Estimate tail approximation

% Copyright (c) 2004-2006 Alessio Botta, Alberto Dainotti, Antonio Pescapè
% Email: {a.botta , alberto , pescape }@unina.it
% DIS - Dipartimento di Informatica e Sistemistica 
% University of Napoli Federico II, ITALY
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[ycdf, xcdf] = cdfcalc(empirical);

% Align ycdf and xcdf (see "help cdfcalc")
ycdf = ycdf(2:length(ycdf));
% Find the value of x where F(x) ~ 0.95
index_a1 = min(find(ycdf >= 0.95));
a1=xcdf(index_a1);
b=length(find(empirical >= a1));
a=length(find(analytical >= a1));
fprintf('b: %d len emp: %d a: %d len an: %d\n', b, length(empirical), a, length(analytical));
% Rescale a
a=a / length(analytical) * length(empirical);
xsi = log2(b / a);