Dave Strouse
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Dave Strouse replied to the topic small sample size and decisions in the forum General 19 years, 10 months ago
Scott –
It appears you desire a prediction interval.
Look at section 44.47 of Jurans Handbook, fifth ed.
Compare spec to limit of prediction interval.

Dave Strouse replied to the topic Shift Happens in the forum General 19 years, 11 months ago
Is the shift 1.5 for all processes? Probably not, probably unique to each process. Are there empirical studies? Hear alusions to them being done at Motorola, but they appear to be “factoids” which I define as something “everyone” knows, but no one can produce the technical report. M. Harry referances a study done in the 1950’s as theoretical…[Read more]

Dave Strouse replied to the topic Sampling size XR Chart in the forum General 19 years, 11 months ago
Rick –
The formula you are proposeing is seriously in error. It is simply the sample size formula for a two sample Z test. However, SPC does not normally use sample sizes consistent with this analysis.
A two sample ttest is a close approximation to the correct method. Please see Wheeler, Montgomery or any other author on construction of OC…[Read more] 
Dave Strouse replied to the topic Control Chart – Sampling Frequency. in the forum General 20 years, 5 months ago
Ernesto –
Are you wanting to continue control charting? Is your questrion how large to make the subgroups? If so –
Get a good basic text on control charting and read up on the idea ofrational subgrouping. Also, look at the ARL for various subgroup sizes. You need to understand the variation in your process and how much undetected drift you can…[Read more] 
Dave Strouse replied to the topic Normal Distributions – why does it matter? in the forum General 20 years, 8 months ago
Stan –
You had posted another messgae earlier this week where you made the same assertions that the control chart was based on between group variation and an asumption of within group variance being zero.
Please stop propagating this error. You are incorrect in these statements. If you have references that say this, please provide them so that…[Read more] 
Dave Strouse replied to the topic Estimated Uncertainties in Control Limits in the forum General 20 years, 8 months ago
Stan –
I’m a little confused by your post.
I’ve not seen control limits for control charting ever expressed as an interval. Could you provide an example?
There is no assumption of zero variance in the subgroups for calculating control limits. Again, can you name a source for this?
Control limits are calculated by estimating the within…[Read more] 
Dave Strouse replied to the topic Industry Std Values Of Sigma for calculating UCL,LCL in the forum General 20 years, 8 months ago
Uma –
This ius very interesting to me. Could you share what document you got this out of. Is it an internal one? If so, whoever wrote it should be retrained. If it is from some other source, could it be some esoteric application?
If it is meant to be control chart guidelines, then it is not,nil,nada, negatory correct for Shewhart type charts…[Read more] 
Dave Strouse replied to the topic Attribute data and sample size in the forum General 20 years, 8 months ago
Sam –
What reason to you have to cull the data?
Would you not use a sample that was for example 100% defective. How about one that is 1% defective? Where is your cutoff to decide what to retain?
If you know of no reason that the sample was abnormally taken, you should use it. If it was abnormal, e.g. the metrology was acting up at the time…[Read more] 
Dave Strouse replied to the topic Industry Std Values Of Sigma for calculating UCL,LCL in the forum General 20 years, 8 months ago
Uma –
In answer to your basic question, NO; these are not correct.
Can you shed some light on where you found such drivel? What industry is supposedly using this information?
Not only are the limits incorrect, but the standard deviation used to compute correct control limits is NOT the standard deviation found in Excel.. An average d…[Read more] 
Dave Strouse replied to the topic Estimate Control Limit from Cp or Cpk in the forum General 20 years, 9 months ago
Ravi
Walter Shewhart wrote “Economic Control of Msanufactured Product” to show a way to calcuolate economic limits for control of a process.
His intent was to show how to balance the two primary risks in manufacturing, i.e. the Alpha nad Beta risks. His conclusion was that 3 sigma limits established using the methodology and fators he…[Read more] 
Dave Strouse replied to the topic Acceptable Quality Level (AQL) in the forum General 20 years, 9 months ago
AQL is the the maximum percent nonconforming(or the maximum number of nonconformities per hundred units) that, for purposes of sampling inspection, can be considered satisfactory as a process average. ANSI/ASQC Standard Z1.41993 paragraph 4.1
This standard also says to interpret AQL in it’s tables as percent nonconformity or no…[Read more] 
Dave Strouse replied to the topic 6Sigma equates to 3.4 DPMO or 6.8 DPMO?? in the forum General 20 years, 9 months ago
Bobby wrote –
The DPMO that determines your sigma level doesn’t distinguish between under spec or over spec. DPMO only looks at defects, as you specify, compared to the total opportunities to create a defect.
I agree with this in general, you can get a measure for sigma in this way that can be used to evaluate the healtha nd later improvement…[Read more] 
Dave Strouse replied to the topic OC Curve and Sampling Plan in the forum General 20 years, 10 months ago
Woey
TomF is correct in stating that acceptance sampling is based on the hypergeometric. This is the distribution to use when sampling without replacement, i.e. taking an item for inspection from the population and NOT putting it back into the population before taking the next item. But the math is difficult because of the large factorials. If y…[Read more] 
Dave Strouse replied to the topic Coefficient of Variation … in the forum General 20 years, 10 months ago
Sridhar –
I hate to flame anyone and perhaps I misunderstood your post, so I will say this as nicely as I can.
If you are using one sigma control limits on any control chart and / or you truely believe that the proportion of points falling within 3 sigma limits is related to the absolute magnitude of the sigma, please seek competant…[Read more] 
Dave Strouse replied to the topic p values in the forum General 20 years, 11 months ago
KMB is exactly right, but maybe a further discussion will help.
Imagine that there is a universe of points from the process you are studying. You take a sample of those to see if you can prove or disprove correlation.
The truth that you are assuming is that there is no or null correlation.
Now you develop some test or way to math…[Read more]