$\backslash$table$[[$Woibools Carse Materials Mitallourcel Crades,The difference between the upper and lower control limits of an $x-$bar chart for a process is $18$ units. Samples of size $7$ have been collected every hour for the past several months. Assuming that $$ is not known, what is the implied average range from the above results $($assume $z=3)?],[$Modiles$,($Round it to two decimal points$)],[$mituTK$,$Table $1.$ Factors for Computing Control Chart Umits $(3$ sigma$)],[\backslash$table$[[$Anteculans$],[($Cliders$)]],$Simple Size, $,$Mean Factor, $A_2,$Upper Range, Du$,$Lower Range, $D_3$