One of our manufacturing facilities asked me to review their SPC X-Bar & R chart for them, and I happened to notice that they are not taking a consistent # of samples per day; therefore the "n" is different for each day.
So when their Range is being calculated, apparently it auto-calculates/changes the D4 constant according to the max number of "n" over the total sample size; so basically, at the end of the month, D4 will have been calculated according to the max subgroup size (4), even if only one day of the month had n = 4 and the rest of the days maybe had n = 3 or n = 2.
Is this OK? I think it's flawed, but I'm getting a headache trying to remember all the rules and calculations that go along with this.
If someone could shed some light on this for me, I would appreciate it greatly!!
So when their Range is being calculated, apparently it auto-calculates/changes the D4 constant according to the max number of "n" over the total sample size; so basically, at the end of the month, D4 will have been calculated according to the max subgroup size (4), even if only one day of the month had n = 4 and the rest of the days maybe had n = 3 or n = 2.
Is this OK? I think it's flawed, but I'm getting a headache trying to remember all the rules and calculations that go along with this.
If someone could shed some light on this for me, I would appreciate it greatly!!