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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Testing a proportion in an online setting
I work in an online security setting. My goal is to detect if the number of locked accounts per time unit is stable or not. I've tried several approaches, detailed below, but I am not satisfied yet. ...
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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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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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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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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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Significance Between Two Percentage Decreases Over Time, Not Mutually Exclusive Variables and Different Sample Sizes
I work in an healthcare improvement department.
I have spent a large amount of time looking for the best way to test this problem for significance and have come up short.
If you take a look at this ...
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perform quality check for imputed data with MICE in R
I'm currently working with the MICE algorithm to impute missing data.
After I did the imputation I wanted to do some kind of quality check of the imputed data set.
There are some suggestions here
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Multiple Comparisons Problem in Control Charts
I have been taught control charts (or Shewhart charts) as a statistical tool to detect processes which are out of control. One particular case was by taking $k$ samples of size $n$ from a process over ...
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Packages for experiment design with multiple levels?
I'm aware of e.g. FrF2 which allows to create 2-level experiment designs. I'm also aware of e.g. gen_design or optFederov which seem to consider multiple levels but when using these I always receive ...
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Why does the X-bar control chart not use control limits from the t distribution
In univariate statistical process control, for the $\bar{X}$ chart with $n$ subgroups each of $m$ samples, we calculate the center line, upper and lower control limits using this equation (1):
Here, $...
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How to find value of omega in CUSUM?
I am trying to understand CUSUM or Cumulative Sum Control Chart.
In the example on above linked Wikipedia page, the formula for high CUSUM value indicating positive ...
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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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CUSUM test for regression model
I guess my question is rather basic. Unfortunately, I still did not manage solve it, although searching for hours.
I have a linear regression model and need to do a CUSUM test for parameter stability.
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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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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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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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What is the probability of at least one of a sum of normally distributed variables being greater than a threshold?
Let's say I have a pencil factory. I don't mind if my pencils are too short, but it's important they're not too long.
My Quality Control department measure each pencil, and their measurement will be ...
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Sampling size determination for small probabilities
I am not sure I could find a clear answer on a correct formula for sampling size determination for finding rare events. Specifically I need to be 99% sure, that failure rate of certain population is ...
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Calculating $\bar c $ in C chart
In studying C chart I came accross this problem in Statistical Process control by Douglas .C.Montgomary.
In this exercise it says:
A control chart is used to control the fraction of non confirming....
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Detecting a trend to increase, in a time series, in real time
Probably, someone who's into technical analysis of share prices eats stuff like this for breakfast. Me, I couldn't devise a theoretically acceptable approach.
I have this thing (private working set, ...
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Mean number of samples before out of control in p-chart
I'm stuck with this problem on quality control and p-charts.
There is a given process, monitored periodically taking samples of 100 items each and counting the number that are defective. In the last ...
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Does the rules "7 or more consecutive points on one side" in Statistical Process Control take in account the amount of data point?
I'm reading on Control chart rules and there is a rule which says: "7 or more consecutive points on one side" in considered "out of control". However, does it take into account the ...
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Does it make sense to calculate Relative Standard Error using population parameters?
Relative Standard Error (RSE) is one of the main measures to assess the quality of survey indicators. If only sample data are available, RSE can be computed using estimated mean $\bar{y}$ and ...
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Which control charts to use to track mortality
I am trying to implement control charts in my organization (healthcare) to track various quality outcomes. The first one i want to do is mortality( # patient deaths/ month(or week) ). I have a data ...
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Find control chart limits based on existing data
I'd like to produce a control chart that tells me if a given process will be within bounds in the future or not.
Currently, the process gives me simple timeseries data, per minute
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Hotelling T squared seemingly useless at detecting a mean shift
I am trying to develop a single-sample Hotelling $T^2$ test in order to implement a multivariate control chart, as described in Montgomery, D. C. (2009) Introduction To Statistical Quality Control, ...
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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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Determining the sampling size ensure defect rate of less than X parts per million at a confidence interval
For quality sampling, I need to ensure a defect rate of less than 5000 parts per million, which is 1/200, at a specific confidence level.
The goal is to have the smallest sample number for the ...
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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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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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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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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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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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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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how to check correctness of data in a database
I want to check the correctness of the data in a database with personal information like birthdate. So a survey is performed by interview and one of the questions was, "what is your date of birth". ...
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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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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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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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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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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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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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How can I check the quality of my probabilities?
I have a data set where the number of accidents are given together with the probability (assumed to be correct) that the number of accidents is less than 4.
The task is to calculate the probabilites ...
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Determining if points out of control - non normal distribution
I am trying to create a process in which I can identify if a process is out of control.
My idea was to do something similar to 6 sigma, where when a point is outside of the mean by +-3 sigma, then ...
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How well does sample average range estimates sigma?
Suppose there are m subgroups of n items each.
The following practice is described by Montgomery in the construction of a $\bar{X}-R$ chart:
Let $\bar{R}$ be the average range of the m subgroups.
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Is there a unified form for Process control and process control analysis
I have done some reading (NIST, JMP, PCS, Juran, some JEDEC, ...) and it appears there are four very common general types of control charts:
Shewhart (X-bar, R, S)
CUSUM (cumulative sum)
EWMA (...
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Quality Control Heat Map
I have data for people about their quality % achieved on a daily basis for last 5 work days, this data is available for one month, say 20 work days.
I have worked out the combinations which is 243. 5 ...
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Recaculating control limits when an out of control situation is displayed
When plotting control charts if a point plots outside the control limits then the reason for that point to plot outside is checked and if an assignable cause is found to be the cause then after ...
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