Module: Statsample::Reliability

Defined in:
lib/statsample/reliability.rb

Defined Under Namespace

Classes: ItemAnalysis, ItemCharacteristicCurve

Class Method Summary collapse

Class Method Details

.cronbach_alpha(ods) ⇒ Object

Calculate Chonbach’s alpha for a given dataset. only uses tuples without missing data



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# File 'lib/statsample/reliability.rb', line 6

def cronbach_alpha(ods)
  ds=ods.dup_only_valid
  n_items=ds.fields.size
  sum_var_items=ds.vectors.inject(0) {|ac,v|
  ac+v[1].variance_sample }
  total=ds.vector_sum
  (n_items / (n_items-1).to_f) * (1-(sum_var_items/ total.variance_sample))
end

.cronbach_alpha_standarized(ods) ⇒ Object

Calculate Chonbach’s alpha for a given dataset using standarized values for every vector. Only uses tuples without missing data



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# File 'lib/statsample/reliability.rb', line 18

def cronbach_alpha_standarized(ods)
  ds=ods.dup_only_valid.fields.inject({}){|a,f|
    a[f]=ods[f].vector_standarized; a
  }.to_dataset
  cronbach_alpha(ds)
end