$\backslash$table$[[$Period$,$Demand,Forecast,Error$],[$January$,130,110,],[$February$,110,115,],[$March$,115,120,],[$April$,125,115,],[$May$,130,125,]]$

A forecasting model has produced the forecasts as shown:

At the end of May the tracking signal would be

$2\mathrm{.}778$

$2\mathrm{.}143$

$1\mathrm{.}333$

$0\mathrm{.}556$