getInitialValues {zipfextR}

Calculates initial values for the parameters of the models.

Description

The selection of appropiate initial values to compute the maximum likelihood estimations reduces the number of iterations which in turn, reduces the computation time. The initial values proposed by this function are computed using the first two empirical frequencies.

Usage

getInitialValues(data, model = "zipf")

Arguments

data

Matrix of count data.

model

Specify the model that requests the initial values (default='zipf').

Details

The argument data is a two column matrix with the first column containing the observations and the second column containing their frequencies. The argument model refers to the selected model of those implemented in the package. The possible values are: zipf, moezipf, zipfpe, zipfpss or its zero truncated version zt_zipfpss. By default, the selected model is the Zipf one.

For the MOEZipf, the Zipf-PE and the zero truncated Zipf-PSS models that contain the Zipf model as a particular case, the \beta value will correspond to the one of the Zipf model (i.e. \beta = 1 for the MOEZipf, \beta = 0 for the Zipf-PE and \lambda = 0 for the zero truncated Zipf-PSS model) and the initial value for \alpha is set to be equal to:

\alpha_0 = log_2 \big (\frac{f_r(1)}{f_r(2)} \big),

where f_r(1) and f_r(2) are the empirical relative frequencies of one and two. This value is obtained equating the two empirical probabilities to their theoritical ones.

For the case of the Zipf-PSS the proposed initial values are obtained equating the empirical probability of zero to the theoretical one which gives:

\lambda_0 = -log(f_r(0)),

where f_r(0) is the empirical relative frequency of zero. The initial value of \alpha is obtained equating the ratio of the theoretical probabilities at zero and one to the empirical ones. This gives place to:

\alpha_0 = \zeta^{-1}(\lambda_0 * f_r(0)/f_r(1)),

where f_r(0) and f_r(1) are the empirical relative frequencies associated to the values 0 and 1 respectively. The inverse of the Riemman Zeta function is obtained using the optim routine.

Value

Returns the initial values of the parameters for a given distribution.

References

Güney, Y., Tuaç, Y., & Arslan, O. (2016). Marshall–Olkin distribution: parameter estimation and application to cancer data. Journal of Applied Statistics, 1-13.

Examples

data <- rmoezipf(100, 2.5, 1.3)
data <- as.data.frame(table(data))
data[,1] <- as.numeric(levels(data[,1])[data[,1]])
initials <- getInitialValues(data, model='zipf')