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Process Screening on I-MR, control limits from within stdev?

Evan_Morris

Simple question. Just curious if the within stdev on the IMR version is used for limits (in the absence of a specified sigma). I'm 99% sure they are, just wanted to confirm, and didn't see this in the process screening report notes.

Accepted answers

tonya_mauldin

If you use the Control Chart Type specification of Indiv and MR in Process Screening, the limits will be calculated using within sigma (average(moving ranges)/d2(n)) the same as Control Chart Builder does. You can find the calculations here:

https://www.jmp.com/support/help/en/15.2/#page/jmp/control-limits-for-individual-measurement-and-moving-range-charts.shtml#ww1157183

All comments

tonya_mauldin

https://www.jmp.com/support/help/en/15.2/#page/jmp/control-limits-for-individual-measurement-and-moving-range-charts.shtml#ww1157183

Evan_Morris

Thanks Tonya, I was 99% sure that was the case but wanted to confirm.

One small other thing I noticed, though I would mention since I have you. Kind of an interesting blip. The Cp in Process Screening uses the within sigma, even if there is a specified sigma, however if you punt to the control chart platform from the Process Screening tool the Cp will use the specified sigma instead of the within.

tonya_mauldin

Please send an example of Cp not matching between Process Screening and Control Chart Builder. I ran the following code and got them to match.

dt=Open("$Sample_Data/Cities.jmp");
dtLimits=New Table( "My Limits",
	Add Rows( 13 ),
	New Column( "Process",
		Character,
		"Nominal",
		Set Values(
			{"OZONE", "NO", "OZONE", "OZONE", "OZONE", "OZONE", "OZONE", "NO", "NO",
			"NO", "NO", "NO", "NO"}
		)
	),
	New Column( "LSL",
		Numeric,
		"Continuous",
		Format( "Best", 12 ),
		Set Values(
			[0.1, 0.02, 0.2, 0.1, 0.1, 0.1, 0.1, 0.02, 0.02, 0.02, 0.02, 0.02, 0.02]
		)
	),
	New Column( "Target",
		Numeric,
		"Continuous",
		Format( "Best", 12 ),
		Set Values(
			[0.2, 0.04, 0.3, 0.2, 0.2, 0.2, 0.2, 0.04, 0.04, 0.04, 0.04, 0.04, 0.04]
		)
	),
	New Column( "USL",
		Numeric,
		"Continuous",
		Format( "Best", 12 ),
		Set Values(
			[0.3, 0.06, 0.4, 0.3, 0.3, 0.3, 0.3, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06]
		)
	),
	New Column( "Grouping",
		Character,
		"Nominal",
		Set Values(
			{"C", "C", "MW", "N", "S", "TX", "W", "C", "MW", "N", "S", "TX", "W"}
		)
	),
	New Column( "Center",
		Numeric,
		"Continuous",
		Format( "Best", 12 ),
		Set Values(
			[0.2, 0.04, 0.15, 0.16, 0.13, 0.2, 0.12, 0.04, 0.026, 0.035, 0.026,
			0.025, 0.02]
		)
	),
	New Column( "Sigma",
		Numeric,
		"Continuous",
		Format( "Best", 12 ),
		Set Values(
			[0.05, 0.01, 0.05, 0.04, 0.06, 0.05, 0.05, 0.01, 0.01, 0.01, 0.02, 0.03,
			0.01]
		)
	),
	New Column( "Importance",
		Numeric,
		"Continuous",
		Format( "Best", 12 ),
		Set Values( [1, 4, 1, 2, 2, 3, 3, 4, 5, 5, 6, 6, 7] )
	),
	Set Label Columns( :Process )
);

obj=dt<<Process Screening(
	Y( :OZONE, :CO, :SO2, :NO ),
	Grouping( :Region ),
	Control Chart Type( "XBar and R" ),
	Subgroup( :State ),
	Minimum Process Length( 1 ),
	Test 2( 1 ),
	Test 3( 1 ),
	Test 4( 1 ),
	Test 5( 1 ),
	Test 6( 1 ),
	Test 7( 1 ),
	Test 8( 1 ),
	Range Limit Exceeded( 1 ),
	Use Limits Table(
		1,
		Data Table( "My Limits" ),
		Process Variables( :Process ),
		Center( :Center ),
		Sigma( :Sigma ),
		LSL( :LSL ),
		USL( :USL ),
		Target( :Target ),
		Importance( :Importance ),
		Grouping( :Grouping),
		Go
	)
);

obj<<Find and Select ("NO", "S");
obj<<Control Charts for Selected Items;

See attached image.

matching.png

Evan_Morris

Tonya

Aha! I figured it out. Very interesting, it bothered me because we had nearly identical scripts.

Here's the trick. Don't specify sigma for 100% of your variables. If you have a couple process variables without a specified sigma, it will use the within for ALL of your variables in process screening. Which makes some sense from a programming perspective as I'm guessing the calculations are happening on the columns, so the specified sigma has some/any nulls in it it defaults to the within column.

Here, I removed the specified sigma for a few of your variables in the script you provided. But the one that makes the control chart does still have a specified sigma, so what shows up in the control chart is different than what is in Process Screening:

dt=Open("$Sample_Data/Cities.jmp");
dtLimits=New Table( "My Limits",
	Add Rows( 13 ),
	New Column( "Process",
		Character,
		"Nominal",
		Set Values(
			{"OZONE", "NO", "OZONE", "OZONE", "OZONE", "OZONE", "OZONE", "NO", "NO",
			"NO", "NO", "NO", "NO"}
		)
	),
	New Column( "LSL",
		Numeric,
		"Continuous",
		Format( "Best", 12 ),
		Set Values(
			[0.1, 0.02, 0.2, 0.1, 0.1, 0.1, 0.1, 0.02, 0.02, 0.02, 0.02, 0.02, 0.02]
		)
	),
	New Column( "Target",
		Numeric,
		"Continuous",
		Format( "Best", 12 ),
		Set Values(
			[0.2, 0.04, 0.3, 0.2, 0.2, 0.2, 0.2, 0.04, 0.04, 0.04, 0.04, 0.04, 0.04]
		)
	),
	New Column( "USL",
		Numeric,
		"Continuous",
		Format( "Best", 12 ),
		Set Values(
			[0.3, 0.06, 0.4, 0.3, 0.3, 0.3, 0.3, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06]
		)
	),
	New Column( "Grouping",
		Character,
		"Nominal",
		Set Values(
			{"C", "C", "MW", "N", "S", "TX", "W", "C", "MW", "N", "S", "TX", "W"}
		)
	),
	New Column( "Center",
		Numeric,
		"Continuous",
		Format( "Best", 12 ),
		Set Values(
			[0.2, 0.04, 0.15, 0.16, 0.13, 0.2, 0.12, 0.04, 0.026, 0.035, 0.026,
			0.025, 0.02]
		)
	),
	New Column( "Sigma",
		Numeric,
		"Continuous",
		Format( "Best", 12 ),
		Set Values(
			[0.05, 0.01, 0.05, 0.04, 0.06, 0.05, 0.05, 0.01, 0.01, 0.01]
		)
	),
	New Column( "Importance",
		Numeric,
		"Continuous",
		Format( "Best", 12 ),
		Set Values( [1, 4, 1, 2, 2, 3, 3, 4, 5, 5, 6, 6, 7] )
	),
	Set Label Columns( :Process )
);

obj=dt<<Process Screening(
	Y( :OZONE, :CO, :SO2, :NO ),
	Grouping( :Region ),
	Control Chart Type( "XBar and R" ),
	Subgroup( :State ),
	Minimum Process Length( 1 ),
	Test 2( 1 ),
	Test 3( 1 ),
	Test 4( 1 ),
	Test 5( 1 ),
	Test 6( 1 ),
	Test 7( 1 ),
	Test 8( 1 ),
	Range Limit Exceeded( 1 ),
	Use Limits Table(
		1,
		Data Table( "My Limits" ),
		Process Variables( :Process ),
		Center( :Center ),
		Sigma( :Sigma ),
		LSL( :LSL ),
		USL( :USL ),
		Target( :Target ),
		Importance( :Importance ),
		Grouping( :Grouping),
		Go
	)
);

obj<<Find and Select ("NO", "S");
obj<<Control Charts for Selected Items;