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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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15 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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7 views

Cusum analysis on a Time-dependent outcome [closed]

Good morning everyone, I need to perform a statistical analysis to study the learning curve of a surgical procedure, by taking into account its performance a time-dependent outcome (Survival). How ...
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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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16 views

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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20 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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17 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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35 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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18 views

How do I find the probability of interference of a shaft and a bearing given their nominal diameter values and their standard deviations?

The exact question is as follows: An assembling of shaft and bearing is made out of shaft manufactured to a specification of 30.00 ± 0.09 mm and bearings are manufactured to a specification of 30.10 ±...
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12 views

np chart: Number of defectives

To control the number of low-fat (defective) oil cans, an $np$ control chart is used, $CL=3$, control limits distant at ± three standard deviations from the center line and $n=150$. How at least ...
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17 views

Sort control charts

I generate monthly and quarterly control charts (XMR) for my organization and am wondering if there exists a method to "sort" control charts so I can filter the 100+ charts to the 10-15 with the "...
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40 views

Control chart control limits for non-normal data

I have the following control chart: To my understanding, control limits are calculated with μ ± 3σ. So basically it's setting boundaries using three standard deviations - and this assumes that the ...
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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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10 views

Dealing with low-resolution sensors for SPC

I want to apply SPC to a data stream coming from a sensor, but the resolution of the sensor is very low. It measures the temperature as an integer, and is almost always at 290 degrees, with a couple ...
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13 views

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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1answer
85 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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43 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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1answer
230 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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1answer
77 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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1answer
47 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
81 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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1answer
114 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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81 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 ...
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30 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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58 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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31 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
46 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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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
180 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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49 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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200 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
63 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
100 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
37 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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223 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
543 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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30 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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274 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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695 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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1answer
494 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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21 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" ...
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1answer
282 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 ...
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2answers
51 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 ...
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1answer
90 views

Comparing new data with old data

I have a use case where I need to check if new data received this week, is different from older data received last week. The data is continuous (basically voltage & power readings from a machine). ...
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31 views

Finding the parameters (n,c) and (N,n,c) in single sampling plans

I'm learning about devising single sampling plans for quality control, and I'd like to learn how to solve for the key parameters. Specifically, I have two related questions. Suppose my boss ...
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1answer
91 views

Control charts constants computation

The various control char constants are usually given for various population sizes, often up to 25. I'd like to have more flexibility; is there à way to compute those instead? Thanks
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4k views

Understanding Defect Rate of Manufacturing Process

(NB: I don't have a very strong background in statistics, so I would not be surprised if this question is naive or the answer is apparent from some rather basic principle.) I'm wondering how I might ...
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181 views

How do you evaluate machine learning model already deployed in production?

so to be more clear lets consider the problem of loan default prediction. Let's say I have trained and tested off-line multiple classifiers and ensembled them. Then I gave this model to production. ...
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34 views

Detecting a significant change in survey answers

I conducted a user study where participants were asked to rate the relevance of search results on the following scale: "very unrelevant", "unrelevant", "relevant" and "very relevant". Every ...
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1answer
38 views

Creating control charts with heteroscedastic variable

I want to make a Shewhart chart with a charting statistic that is symmetrical but heavy-tailed (e.g., Cauchy). Does there exist anything like a Z-score with a heteroscedastic variance model? For ...
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1answer
26 views

Tolerance Interval for a Non-Random Sample

I have a sample of measurements representing the number of defects at a certain time point. I have 74 measurements over the past year. In addition, I have a limit of the maximum number of defects ...