Stack Exchange Network

Stack Exchange network consists of 174 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.

Visit Stack Exchange

Quality Control relates to statistical methods used for monitoring, maintaining, and/or improving either the statistical quality control of a process or the capability of a process.

0
votes
0answers
6 views

statistical check if minimum spec values are exceeded

Detailed fatigue data are available for our materials. These describe the failure probability at defined stress levels with a known log-normal distribution. So I know mean and standard deviation. Now ...
1
vote
0answers
9 views

SPC charts - Do these determine signifcant variation?

My understanding of SPC charts is that they simply pin point when a datum is outside of 3 Z scores (dependent on how you define your confidence intervals). When a data point is outside of these ...
0
votes
0answers
7 views

Average of counts - which control chart to use?

I have data coming from a lab. Each test examines how many bacteria units are in the lab (counts). The examination was done 3 times a day, during a period of 16 days. The quality specifications of the ...
0
votes
0answers
12 views

Comparing checklists with different numbers of questions

I have some products that I need to design a packaging QC checklist for. Ideally I would like to be able to compare the results of these checklists between products. The problem is that not all ...
0
votes
0answers
16 views

AQL vs AOQL Sampling

What is the benefit of switching from a sampling plan based on AQL to a sampling plan based on AOQL? I believe I understand the definitions of each, such as stated here, but I do not understand the ...
0
votes
0answers
8 views

Schewhart X-bar Chart: Disadvantage of using subgroup of size 1?

I understand the correct procedure for creating X-bar charts but find myself wondering where the disadvantage stems from in reducing the subgroup size from 4 or 5 down to 1. I would think that if I ...
0
votes
0answers
6 views

Control Limit Estimates: How are subgroups utilized in univariate data?

I see a lot of mention of subgroups when reading about control limits but am wondering how they're supposed to be utilized. E.g., if a factory produces widgets and wants those widgets to sustain a ...
0
votes
0answers
7 views

IMR / Shewhart Charts and Missing Data?

I've been asked to modify a set of IMR charts so they 'show' lots with missing data. I'm interpreting the request to mean display the lot on the X-axis with other lots and somehow leave a gap in the ...
1
vote
1answer
27 views

What is meant by subgroup?

I am learning about control charts but I am unable to wrap my head around the concept of subgroups. What are subgroups? I understand that they are group of homogeneous data, but why do we need to ...
0
votes
0answers
12 views

Identify whether my NEW data is having some new pattern(s) or trend(s) from the OLD data in REAL time

After going into production the model starts predicting in Real Time. Therefore it becomes the need to handle the scenarios that may arise in Real Time. If we are planning to use the REAL TIME data ...
0
votes
0answers
5 views

Identify whether my NEW data is having some new pattern(s) or trend(s) from the OLD data

After going into production the model starts predicting at Real Time. Therefore it becomes the need to handle the scenarios that may arise in Real Time. If we are planning to use this data for ...
0
votes
0answers
11 views

Comparing Distributions - Continuous Predictor with Binomial Response

I am trying to determine whether there is a statistical difference between a samples distribution to a known population. The predictor is velocity, and the response is Complete Penetration/Partial ...
0
votes
0answers
8 views

Is there a systematic behaviour/change?

don't know how to call the title so please excuse the rather general naming. I've a lot devices and each has it's own specific voltage which I've histogrammed. After the same treatment (meaning, each ...
0
votes
0answers
18 views

Data transformation and tolerance intervals

I need to construct tolerance limits for some data. I am doing the next approach: I have n data points I transform the data to normal data using a Johnson transformation. I compute the desired ...
0
votes
0answers
6 views

Area between upper control limit and mean… Is it meaningful?

I'm trying to come up with a metric that assesses the general "health" associated with some system. I don't have the right data calculated to come up with a classification model or anything like that. ...
0
votes
0answers
23 views

Judging when to use Individual moving range control charts vs X-bar S or R control charts for subgroups?

I obtain temperature data every 30 minutes for 6 hours or so. I want to chart this data for trends, special causes etc for 80 similar instances. I've read that if the reading is over a single ...
0
votes
0answers
126 views

Johnson Transformation Inverse

I am using the Johnson package in R, I need after the Johnson transformation do some inference using the results of Normal distribution, after that I need to go back to the original data. I have ...
0
votes
0answers
9 views

Calculating standard deviation from succesive differences identifies inherent versus externally caused variation?

I am reading Six Sigma For Dummies by Gygi and Williams, 2012. In the book (chapter 9, page 125) it says that calculating standard deviation by taking the average of the absolute successive ...
0
votes
0answers
9 views

Runs indicating a trend in statistical process control?

Suppose I have a factory machine that outputs widgets. They are supposed to be all the same weight, but in reality fluctuate slightly in weight from widget to widget. This fluctuation could be random, ...
1
vote
1answer
69 views

Non-normal distributions vs data transformation

How would an individual decide on treating Non-normal data for, e.g. control charts; data transformation vs Non-normal distributions such as Weibull? I have access to Minitab but the decision point ...
0
votes
1answer
14 views

XmR Chart Control Limits - Why Moving Range - Why Not population 3sigma

Why do we compute the control limits for the individuals or XmR chart with an estimated sigma, 2.66 x mR(avg)? I understand that the overall population sigma is different. But why is one more ...
1
vote
1answer
40 views

If the population size was large enough that the sample size required to represent it is too large to be practical, what should be done?

i just started working as a quality control engineer at a poultry company, one of my first duties is to design a mechanism to ensure the quality of the end product (the Chick) and processes leading up ...
0
votes
2answers
47 views

How to calculate reliability (or confidence of no more errors) after a large run with no errors?

I have a (software) system which was failing, with a particlular failure mode, about 0.5% of the time. This was low enough that it was not noticed during inital development. After performing several ...
0
votes
1answer
48 views

monitoring variation for a process with a non-constant average

We use control charts in our lab to monitor process variation, and I'm trying to reconfigure the chart for one of our methods, so data is better represented. For the method in question, we use a ...
1
vote
0answers
35 views

teaching over-calibration (from process control) to non-stats folks

When I took my courses in process control, they talked about "over-calibration", which is repeatedly tuning a process to attempt to reduce natural cause variation, as something that increased overall ...
0
votes
0answers
15 views

SPC - do the Nelson rules change if the control limits are changed from the “default” of 3 standard deviations?

I am aware that, in SPC, one can theoretically chose different control limits based on the number of standard deviations one wishes to use. If one uses a control chart with control limits with fewer ...
0
votes
0answers
37 views

How to find probability of out of control point

I'm trying to analyze the temperature of the industrial motor. I have assumed that temperature is normally distributed. Based on this assumption I had estimated UCL for the Temperature. Now I want to ...
0
votes
0answers
44 views

How to calculate the T² statistic? multivariate process control

Following is a multivariate control quality problem in which I have to use Hotelling's T² statistic, and I need help on how to use the formula. An explanation showing how to solve by hand for just one ...
0
votes
0answers
11 views

How to spot LTPD in OC curve if it is more than 30%. Becuase in the proportion of defective (p) values are showing only until0.2?

How to construct OC Curve if N=800,n=10, c=2. Suppose 10% nonconforming products are acceptable to the manufacturer and 30% nonconforming products are not acceptable.Please help.
0
votes
0answers
9 views

Detect change in the mean of output from an old factory machine

I have an old complicated factory machine that whacks metal to produce a widget every minute. It is very important that the mean length of a widget stays the same over time, even though this varies ...
0
votes
0answers
10 views

What's the most appropriate way to define limits of a control substance in an assay/experiment?

I'm looking for the best way of defining limits for values of a control substance employed in an assay. Assume we are doing repetitively an experiment/assay and we want to know when the experiment ...
2
votes
0answers
53 views

Is it possible to use statistics to detect a rising temperature scenario?

It is required to monitor temperature data from a manufacturing environment. The particular use case is to detect if the temperature is rising beyond normal values to enable timely corrective action. ...
1
vote
0answers
28 views

Validating my thinking

I'm pretty sure I'm seeking standard deviation and finding the mass of my 'widgets' that exist outside my SD. Here's the context. I just sorted a bunch of widgets into two piles. One pile is known ...
1
vote
1answer
41 views

Detect statistically significant decrease in event rate

Pardon the novice question, but I really don't know what I'm supposed to be searching for on this one. I am monitoring a system for raw events. I can identify the "type" or "kind" of an event, but ...
0
votes
1answer
654 views

X-bar R charts with varying subgroup size

I am using control limits to check if a process is going out of control or not and detect the mean shifts over time. I am aware of how to apply the control limits, but not really sure about the ...
1
vote
1answer
87 views

Is there a Status Machine Chart?

I find this type of chart quite often, but I don't know if it is known by another specific name: It looks like a 100% stacked bar chart, but colors repeat and it is actually different. It may be ...
1
vote
0answers
36 views

Estimating a tail probability based on the sample mean and standard deviation

I want to know if a sub-population has the same upper quantile as a parent population. Assume that in the parent population, proportion $1-\alpha$ of the data lies below threshold $k$. I can observe ...
2
votes
0answers
119 views

Poisson Distribution Process Control Limits

I am measuring the number of events that happen within n minute intervals for a group of 100 stores. My problem is, I am looking for a way to calculate a threshold for the acceptable number of times ...
2
votes
1answer
43 views

Nelson Rules - what if points are only just on one side of the mean?

This question relates to the Nelson rules for statistical process control. Does rule 2 - nine points in a row on the same side of the mean - apply if one or two of those points are very narrowly on ...
0
votes
1answer
78 views

Control limits for Xbar chart

I am flirting with the idea of using econometrics and regression to improve upon the classical SPC control charting techniques. Hopefully with the end of goal of doing trend analysis on mean and ...
0
votes
1answer
31 views

Quasi-experimetal Research is it necessary a control group?

A short-term entrepreneurship course is gonna be implemented in one city in 10 schools. I would like to stablish a quasi-experimental research in order to measure the impact of this course on skills, ...
0
votes
0answers
93 views

I/MR control chart center line

I'm doing a small comparison of methods for estimating sigma in control charts for specific industry data and got stuck with a moving range chart. Most of apps and researches do use average moving ...
6
votes
2answers
216 views

“Random variable” vs. “random value” (when translating from Russian into English)

In the introductory part of a Russian document I was translating, there was a simplified description of a control chart-based quality management system. The description contained this sentence in ...
1
vote
1answer
300 views

Statistical Quality Control Data Gathering

Data has been gathered for a specific analytical test method to start SQC. The issue is that when the data is tested for normality, using Anderson Darling, the null hypothesis can not be rejected. -...
1
vote
1answer
26 views

How to find the time to failure of a component using mean failure time

Is there a way to find the time it will take a component to fail if we know the average failure time. For example a bulb has a mean failure time of 100 hours but is working even after 200 hours. Can ...
1
vote
0answers
172 views

Imputing missing values and quality control of hourly weather data

I have hourly weather data for a number of weather stations in radius of some 300 kilometers, for second half of the last century. Missing values range from few hours to a whole month. I am working ...
0
votes
0answers
286 views

Why is Bayes Success Run Theorem independent of total “population” size?

In Bayes success-run theorem (below), total "population" size is not a parameter. Why is this? $$R = (1-C) ^{ (1/n)}$$ where: $R =$ Reliability (or probability of success), $C =$ confidence level, ...
3
votes
1answer
286 views

One-class KNN for Quality Control

I've come across this paper https://uta-ir.tdl.org/uta-ir/bitstream/handle/10106/1827/Sukchotrat_uta_2502D_10083.pdf?sequence=1&isAllowed=y] where it is described a k-Nearest Neighbors Data ...
0
votes
0answers
17 views

Control Chart factor $c_4$

I have found several authoritative sources with differing methods for calculating the control chart factor $c_4$. Are these methods truly equivalent, and if they are not, which method is the "real" ...
0
votes
2answers
50 views

Applying Sample Sizing To Performance Monitoring?

Let's say you have a machine that runs on an algorithm with inputs that can be modified. Every 3 months, that machine produces 40,000,000 widgets. You can monitor if the widgets are being made ...