EWMA CONTROL CHARTS

Basic usage

ewma = QCC::EWMA.new
ewma.group_size = 5
i = 0
File.open("diameters.txt") do |f|
  f.each_line do |l|
    group = l.split.map {|e| e.to_f}
    chart = ewma.update(group)
    cl    = ewma.control_limits
    puts "#{i+=1} #{chart[:xn]}, #{chart[:z]}, #{cl.inspect}"
    ewma.calibrating = (i < 25)
  end
end

Requirements

No other gems/libraries are required, although you must have the Flotr library installed to have examples work:

sudo gem install pbosetti-flotr

To Do

At the moment, global mean and standard deviation are routinely updated after adding each group, including the calibration phase. This is a different behavior wrt the standard EWMA charts, where the calibrating phase is used to get a value for those indicators. This is currently due to the recursive formula adopted for mean and standard deviation calculation, and will be corrected in the future revisions.