Questions tagged [quality-control]

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. Use for questions about 6 sigma.

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13 views

Can CUSUM be used to detect anomalies in clustered data?

I am looking at approaches to detect anomalies in network traffic in terms of packets being sent with a very short inter-arrival time (IAT). Said differently, the longer (possibly) uniform IATs would ...
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Calculating upper and lower confidence limits in R (99% and 95%) for a funnel plot that does NOT look at rate

I am a programmer but not a statistician. I am trying to create a funnel plot in R that looks at average chest tube duration for each individual hospital in hours. The x-axis has the total number of ...
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7 views

Effectiveness checking of control plan / implemented corrective actions

I have a question about statistics for a manufacturing environment. I work in a heavily regulated environment for medical devices, as a manufacturing engineer, but statistics is not my strong suite. ...
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What do the letter and the subscript in the Shewhart's control chart constants ($A_2, A_3, c_4, c_5, \ldots$) mean?

The expected value of the sample standard deviation is $$E(s) = c_4(n)\sigma$$ where $$ c_4(n) = \sqrt{2\over n-1}{\Gamma({n\over2})\over\Gamma({n-1\over2})} $$ The page on Wikipedia led me ...
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How to measure the repeatability in an experiment design?

I'm performing a design of experiment (CCD) with 6 factors: H2, CO, C2H2, ...
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28 views

Find a single-sampling plan for which $p_1=0.02$, $\alpha=0.01$,$p_2=0.06$ and $\beta=0.10$

I am not sure where to start. My previous lecture gave very limited information on what to do here, so my current toolbox only consists of these formulas: $$1-\alpha=\sum_{d=0}^{c} {n \choose d}p_1^d(...
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19 views

How I can simulate autocorrelated data time varying mean in R?

Actually I am working on SQC and I want to fit an AR(1) model to the autocorrelated data with changing mean and use the residuals as charting statistic (shewhart, EWMA, CUSUM) to study the small ...
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5 views

Tolerance Interval of large Time Series data

I have a time series Temperature data coming from a sensor which is collected at a irregular frequency. The data size is ~100K. I have to find the Lower and Upper Bounds for this data to find the ...
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2 views

Ideal Sample Size and Quality Scores

How do I calculate ideal sample size for an activity? I am working on a quality assurance project. The project involves review of samples and preparation of reports. These reports are then quality ...
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124 views

What is the rationale for the rules for detecting an out of control process in Statistical Process Control?

Statistical Process Control (SPC) can be used to determine if a process is "in statistical control". A common tool for SPC is the "mean control chart" -- essentially a time series of sample means ...
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21 views

Sample Size Estimate of Proportion

College admission office manually processes 1,000 applications in a month. We want to take a sample from this 1,000 to see how many applications were processed correctly (data entry errors, typos, ...
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Quantifying agreement between datasets from different methods of measurement

I have two datasets, A and B. Both ultimately come from the same source, both have the same data attributes, but the datasets use different measurement methods to derive the attributes. We want to ...
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46 views

Correctly Specifying C=0 Sampling Plan

I looking for a little help with how to go about specifying/requiring the use of a C=0 sampling plan on an engineering drawing. In the past, we have typically been able to refer to a standard, for ...
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16 views

Part selection in gauge R&R study

I'd like to conduct a gauge R&R study for product control. However, I am struggling with how to select the parts. To me it seams as if every author has his/her own way to select the parts for the ...
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10 views

How to test simulations of control charts

I coded up a Shewart control chart and CUSUM control chart for detecting mean shifts in a time series. I can create a toy data stream with preplanned change points works as a one-off but is there any ...
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19 views

Using the sum of the absolute residuals divided by the sample sum as a normalized goodness-of-fit estimator

I am trying to make an automatic method to do a peak fitting to some spectra, using multiple least squares. I would like to be able to compare the quality of the fit between several different cases. ...
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Is it possible to get equivalences of Western Electric Rules in non normal distribution with percentile values?

I am not a statistician and I am a bit lost with these concepts, but I will try to explain my situation. I have a non-normally distributed data, and I am trying to apply the Western Electric Rules to ...
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When and how to re-evaluate deployed models

There are a lot questions about how to train a model (family) or how to tune hyperparmeter. But there are surprisingly few question about how to monitor or evaluate a model already in production (...
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28 views

Forecasting Process with Limited Historical Data and High Variance

I have a general inquiry regarding a project I am working on. I cannot reveal too much, but I would like to gauge the community here and hopefully be pointed towards the right direction in terms of ...
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22 views

Statistical Process Control with Big Data

At work one of our projects has been to monitor a certain type of mechanical system and identify anomalous behavior. Luckily for us we have access to 5+ years of system performance across many ...
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113 views

Quantile (Inverse Cumulative Density) Function for Hypergeometric Distribution

The hypergeometric distribution arises from sampling without replacement. The similar binomial sampling distribution assumes replacement. Hypergeometric distributions are commonly used in quality ...
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12 views

Product Yield in Capability Analysis

I couldn't find a 6 sigma tag so please let me know if this is posted in the wrong forum. I'm performing a capability analysis on a manufacturing process. However my process output is Pass/Fail. This ...
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Which control chart can use for monitoring of covariance matrix of cofficient?

I have a series of panel data, which includes p cross section and t time. I estimated the covariance matrix of coefficients for periods for periods. Now my data includes the individual period and the ...
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230 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 ...
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53 views

t-test with drift in reference distribution

I wonder whether I am behaving correctly with my Student t-test exercise. I am continuously testing a small data sample of metal-detector antenna readings against a calibration sample distribution. ...
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313 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 ...
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87 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 ...
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48 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 ...
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2answers
145 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 ...
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122 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 ...
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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 ...
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61 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 ...
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60 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. ...
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32 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 ...
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1answer
53 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 ...
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1answer
1k 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 ...
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1answer
221 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 ...
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65 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 ...
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232 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 ...
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1answer
79 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 ...
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1answer
102 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 ...
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1answer
40 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, ...
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227 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 ...
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1answer
597 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. -...
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1answer
32 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 ...
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307 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 ...
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865 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, ...
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550 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 ...
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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" ...
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1answer
351 views

Mathematical approach for finding baseline shifts in data

I have a raw signal from a sensor which is attached in this question . The x-axis is time, and the y-axis is the signal response. As you can see from the data, the baseline of the noisy signal shifts ...