NonlinearFit {GB2}

Fitting the GB2 by Minimizing the Distance Between a Set of Empirical Indicators and Their GB2 Expressions

Description

Fitting the parameters of the GB2 distribution by optimizing the squared weighted distance between a set of empirical indicators, i.e. the median, the ARPR, the RMPG, the QSR and the Gini coefficient, and the GB2 indicators using nonlinear least squares (function nls from package stats).

Usage

nlsfit.gb2(med, ei4, par0=c(1/ei4[4],med,1,1), cva=1, bound1=par0[1]*max(0.2,1-2*cva), 
bound2=par0[1]*min(2,1+2*cva), ei4w=1/ei4)

Arguments

med

numeric; the empirical median.

ei4

numeric; the values of the empirical indicators.

par0

numeric; vector of initial values for the GB2 parameters a, b, p and q. The default is to take a equal to the inverse of the empirical Gini coefficient, b equal to the empirical median and p = q = 1.

cva

numeric; the coefficient of variation of the ML estimate of the parameter a. The default value is 1.

bound1, bound2

numeric; the lower and upper bounds for the parameter a in the algorithm. The default values are 0.2*a_{0} and 2*a_{0}, where a_{0} is the initial value of the parameter a.

ei4w

numeric; vector of weights of to be passed to the nls function. The default values are the inverse of the empirical indicators.

Details

We consider the following set of indicators A = (median, ARPR, RMPG, QSR, Gini) and their corresponding GB2 expressions A_{GB2}. We fit the parameters of the GB2 in two consecutive steps. In the first step, we use the set of indicators, excluding the median, and their corresponding expressions in function of a, ap and aq. The bounds for a are defined in function of the coefficient of variation of the fitted parameter \hat(a). The nonlinear model that is passed to nls is given by:

\sum_{i=1}^4 c_i (A_{empir,i}-A_{GB2,i}(a,ap,aq))^2,

where the weights c_i take the differing scales into account and are given by the vector ei4w. ap and aq are bounded so that the constraints for the existence of the moments of the GB2 distribution and the excess for calculating the Gini coefficient are fulfilled, i.e. ap \ge 1 and aq \ge 2. In the second step, only the the parameter b is estimated, optimizing the weighted square difference between the empirical median and the GB2 median in function of the already obtained NLS parameters a, p and q.

Value

nlsfit.gb2 returns a list of three values: the fitted GB2 parameters, the first fitted object and the second fitted object.

Author(s)

Monique Graf and Desislava Nedyalkova

See Also

nls, Thomae, moment.gb2

Examples

# Takes long time to run, as it makes a call to the function ml.gb2

## Not run: 
library(laeken)
data(eusilc)

# Personal income
inc <- as.vector(eusilc$eqIncome)

# Sampling weights
w <- eusilc$rb050

# Data set
d <- data.frame(inc, w)
d <- d[!is.na(d$inc),]
   
# Truncate at 0
d <- d[d$inc > 0,]
inc <- d$inc
w   <- d$w

# ML fit, full log-likelihood
fitf <- ml.gb2(inc, w)$opt1

# Estimated parameters
af <- fitf$par[1]
bf <- fitf$par[2]
pf <- fitf$par[3]
qf <- fitf$par[4]

gb2.par <- c(af, bf, pf, qf)

# Empirical indicators
indicEMP  <- main.emp(inc, w)
indicEMP <- c(indicEMP[1],indicEMP[3:6])
indicE <- round(indicEMP, digits=3)

# Nonlinear fit
nn <- nlsfit.gb2(indicEMP[1,3:6],indicEMP[3:6])
an <- nn[[1]][1]
bn <- nn[[1]][2]
pn <- nn[[1]][3]
qn <- nn[[1]][4]

# GB2 indicators
indicNLS <- c(main.gb2(0.6, an, bn, pn, qn)[1], main.gb2(0.6, an, bn, pn, qn)[3:6])
indicML <- c(main.gb2(0.6, af, bf, pf, qf)[1], main.gb2(0.6, af, bf, pf, qf)[3:6])
indicN <- round(indicNLS, digits=3)
indicM <- round(indicML, digits=3)

# Likelihoods
nlik <- loglp.gb2(inc, an, bn, pn, qn, w)
mlik <- loglp.gb2(inc, af, bf, pf, qf, w)

# Results
type=c("Emp. est", "NLS", "ML full")
results <- data.frame(type=type,
        median=c(indicE[1], indicN[1], indicM[1]),
        ARPR=c(indicE[2], indicN[2], indicM[2]),
        RMPG=c(indicE[3], indicN[3], indicM[3]),
        QSR =c(indicE[4], indicN[4], indicM[4]),
        GINI=c(indicE[5], indicN[5], indicM[5]),
        likelihood=c(NA, nlik, mlik),
        a=c(NA, an, af), b=c(NA, bn, bf) ,p=c(NA, pn, pf), q=c(NA, qn, qf))


## End(Not run)