If u want for variable SPC, then actually some times it is based on customer requirements for no of sample like 80 or 125.....for standard SPC we have to collect 125 pcs in 25 subgroups at the time interval of 2 hr (can select time as per your process length)
After collection of sample measure any dimension or other parameter for which u want to do analysis and directly paste data into SPC standard format...........SPC is always mandatory for critical dimensions..........
