A high$-$voltage power supply should have a nominal

output voltage of $350$ V$.$ A sample of four units is selected each day and tested for process$-$control pur$-$ poses. The data shown in Table $6$E$\mathrm{.}3$ give the differ$-$ ence between the observed reading on each unit and the nominal voltage times ten; that is$,$

xi $=($observed voltage on unit i $-350)10$

$($a$)$ Set up x and R charts on this process. Is the

process in statistical control?

$($b$)$ If specifications are at $350$ V $\backslash$pm $5$ V$,$ what can you

say about process capability?

$($c$)$ Is there evidence to support the claim that volt$-$

age is normally distributed?