# R/indicator_skewness.R In spatialwarnings: Spatial Early Warning Signals of Ecosystem Degradation

#### Documented in indicator_skewness

 #' @title Skewness indicator
#'
#' @description This functions computes the spatial skewness of spatial data.
#'   It also computes a null value obtained by randomizing
#'   the matrix.
#'
#' @param input A matrix or a list of matrices. The matrix
#'   values can be logical, with \code{FALSE} (empty) or \code{TRUE} (occupied)
#'   values. The entries can also be continuous (like NDVI or EVI data).
#'
#' @param subsize  Dimension of the submatrix used to coarse-grain the
#'   original matrix. This must be an integer less than size of the full
#'   matrix. Coarse-graining reduces the size of the matrix by a factor
#'   \code{subsize} in each dimension of the matrix. Skewness is calculated
#'   on the coarse-grained matrix.
#'
#' @param absolute Should the function return the absolute value or raw value
#'   of skewness ?
#'
#' @param nreplicates Number of replicates to produce to estimate null
#'   distribution of index.
#'
#' @return A list (or a list of lists if input was a list of matrices) with
#'   components:
#'     \itemize{
#'       \item value: Spatial skewness of the matrix
#'     }
#'   If nreplicates is above 2, then the list has the following additional
#'   components :
#'     \itemize{
#'       \item null_mean: Mean skewness of the null distribution
#'       \item null_sd: SD of skewness in the null distribution
#'       \item z_score: Z-score of the observed value in the null distribution
#'                          (value minus the null mean and divided by null
#'                          standard deviation)
#'       \item pval: p-value based on the rank of the observed skewness
#'                       in the null distribution. A low p-value means that
#'                       the indicator value is significantly higher than the
#'                       null values.
#'     }
#'
#' @details
#'
#' Spatial skewness is a measure of fluctuations in space; specifically, it
#' measures if fluctuations are getting biased (skewed) in one direction. Based
#' on the theory of critical slowing down, when systems approach critical
#' points they are expected to show increased fluctuations in space. Thus,
#' increasing spatial skewness is proposed as an early warning signal of
#' impending critical transitions.
#'
#' Computing spatial skewness is straightforward. However, detecting trends of
#' skewness that correspond to critical slowing down can be tricky, especially
#' if data come from discrete classification of state variable.
#'
#' For example, many high resolution spatial data are classified as FALSE (empty)
#' or TRUE (occupied by plant). In such cases, spatial skewness captures just
#' the skewness in data, but not that of spatial structure.
#' To resolve the issue, this function employs a method called coarse-graining,
#' proposed in Kefi et al (2014), and described in detail in
#' Sankaran et al. (2017). One must specify a subsize above one for
#' binary valued data sets to obtain meaningful values.
#'
#' \code{subsize} has to be an integer. It has to be less than or equal to
#' half of matrix size (N). \code{subsize} must also be preferably a
#' divisor of N. If it is not a divisor of N, the remainder rows and columns
#' are discarded when computing coarse-graining matrices.
#'
#' Null model evaluations are also done on coarse-grained matrices.
#'
#' @references
#'
#' Guttal, V., and Jayaprakash, C. (2009). Spatial variance and
#' spatial skewness: leading indicators of regime shifts in spatial
#' ecological systems. Theoretical Ecology, 2(1), 3-12.
#'
#' Kefi, S., Guttal, V., Brock, W.A., Carpenter, S.R., Ellison, A.M.,
#' Livina, V.N., et al. (2014). Early Warning Signals of Ecological
#' Transitions: Methods for Spatial Patterns. PLoS ONE, 9, e92097.
#'
#' Sankaran, S., Majumder, S., Kefi, S., and Guttal, V. (2017). Implication of
#' being discrete and spatial in detecting early warning signals of regime
#' shifts. Ecological indicators.
#'
#' @examples
#'
#' data(serengeti)
#' \dontrun{
#' indicator_skewness(serengeti)
#' }
#'
#'@export
indicator_skewness <- function(input,
subsize     = 5,
absolute = TRUE,
nreplicates = 999) {

check_mat(input) # checks if binary and sensible

if ( is.list(input) ) {
# Returns a list of lists
return( lapply(input, indicator_skewness,
subsize, absolute, nreplicates) )
} else {

# We choose the function depending on whether we want the absolute value
#   of skewness or its raw value.
if ( absolute ) {
indic_tmp <- raw_abs_skewness
} else {
indic_tmp <- raw_skewness
}

# We alter the chosen function above depending on whether we want
#   coarse_graining or not.
if ( subsize > 1 ) {
indicf <- with_coarse_graining(indic_tmp, subsize)
} else {
indicf <- indic_tmp
}

# Compute and return the indicator
return( compute_indicator_with_null(input, nreplicates, indicf) )

}
}

raw_abs_skewness <- function(mat) {
abs(raw_skewness(mat))
}


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spatialwarnings documentation built on June 18, 2018, 1:01 a.m.