# Using Standard Deviation to Set Nominal Target

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This topic contains 2 replies, has 3 voices, and was last updated by Chris Seider 3 months, 2 weeks ago.

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- August 20, 2019 at 9:11 am #241175

Ahiru-sanParticipant@Ahiru-san**Include @Ahiru-san in your post and this person will**

be notified via email.**How can I use standard deviation to target nominal?**Say my nominal width dimension is 90 mm.

My Lower Spec Limit is 80 mm and my Upper Spec Limit is 100 mm.

My standard deviation is 2 mm.

So, my Lower Control Limit is 84 mm while my Upper Control Limit is 96 mm (that is 3 standard deviations either way). I have good process capability, but it with that much variation (+/- 6 mm),

**how do I target nominal?****Please check my logic here.**A. Zero Standard Deviations = If I say we cannot start until I measure a nominal width of 90, I know that I will have a lot of setup time because the process normally varies from 84-96. This is normal variation. So that won’t work.

B. 1 Standard Deviation = If I start anywhere from 88 to 92. 88-6= 82 and that is inside my LSL. 92+6=98 and that is inside my USL. So, nominal +/- 1 standard deviation will work, but may be require additional setup time.

C. 2 Standard Deviations = I can start anywhere from 86 to 94 that means 86-6=80 and that just reaches my LSL. While 94+6=100 and that also is technically in spec. So, that would work, but just barely.

D. 3 Standard Deviations = If I use my control limits of 84-96, I know the process may vary normally from a low of 84-6=78 (which is below my tolerance of 80), and it could go as high as 96+6=102 (which is above my USL of 100). Okay, so that won’t work.

**Is this a reasonable approach?****Is there some standard way of doing this?****Have I missed something?**0August 20, 2019 at 2:58 pm #241181

Mike CarnellParticipant@Mike-Carnell**Include @Mike-Carnell in your post and this person will**

be notified via email.@ahiru-san Where inside the spec is the most profitable place for your company to produce the product? You might want to understand the Taguchi Loss Function.

1August 20, 2019 at 8:34 pm #241198

Chris SeiderParticipant@cseider**Include @cseider in your post and this person will**

be notified via email.Interesting query.

Have you put together a mathematical model to help answer your questions and @mike-carnell has given you a hint to an approach.

keep in mind you’re making lots of assumptions…would you be using an Xbar chart–if so I’d doubt your CL’s would be as you’ve concluded.

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