Class: SpatialStats::Local::GetisOrd

Inherits:
Stat
  • Object
show all
Defined in:
lib/spatial_stats/local/getis_ord.rb

Overview

GetisOrd's G and G* statistics compute the spatial autocorrelation of a variable, x. G computes the ratio of spatially lagged x to the sum of all other x's except xi for every entry. G* does the same calculation but includes xi in the spatial lag and denominator.

Instance Attribute Summary collapse

Attributes inherited from Stat

#field, #scope, #weights

Instance Method Summary collapse

Methods inherited from Stat

#crand, #expectation, #mc, #mc_bv, #quads, #summary, #variance, #y=, #z_score

Constructor Details

#initialize(scope, field, weights, star = nil) ⇒ GetisOrd

A new instance of GetisOrd

Parameters:

  • scope (ActiveRecord::Relation)
  • field (Symbol, String) —

    to query from scope

  • weights (WeightsMatrix) —

    to define relationship between observations in scope

  • star (Boolean) (defaults to: nil) —

    to preset if star will be true or false. Will be calculated otherwise.



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# File 'lib/spatial_stats/local/getis_ord.rb', line 20

def initialize(scope, field, weights, star = nil)
  @scope = scope
  @field = field
  @weights = weights
  @star = star
  calc_weights
end

Instance Attribute Details

#star ⇒ Object

Returns the value of attribute star.



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# File 'lib/spatial_stats/local/getis_ord.rb', line 27

def star
  @star
end

#x ⇒ Array Also known as: z

Values of the field queried from the scope

Returns:

  • (Array) 


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# File 'lib/spatial_stats/local/getis_ord.rb', line 64

def x
  @x ||= SpatialStats::Queries::Variables.query_field(@scope, @field)
end

Instance Method Details

#groups ⇒ Array

Computes the groups each observation belongs to. Potential groups for G are:

H

High

L

Low

Group is high when standardized z is positive, low otherwise.

Returns:

  • (Array) —

    groups for each observation



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# File 'lib/spatial_stats/local/getis_ord.rb', line 50

def groups
  z.standardize.map do |val|
    if val.positive?
      'H'
    else
      'L'
    end
  end
end

#star? ⇒ Boolean

True if G* is being used, false if G is being used. If no value is passed in the constructor, it will be determined based off of the trace of the weights.

Returns:

  • (Boolean) —

    of star



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# File 'lib/spatial_stats/local/getis_ord.rb', line 75

def star?
  if @star.nil?
    @star = weights.dense.trace.positive?
  else
    @star
  end
end

#stat ⇒ Array Also known as: g

Computes the G or G* statistic for every observation in x.

Returns:

  • (Array) —

    of autocorrelations for each observation.



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# File 'lib/spatial_stats/local/getis_ord.rb', line 34

def stat
  x.each_with_index.map do |_x_val, idx|
    stat_i(idx)
  end
end