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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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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Anomalies detection algorithms

I would like to detect drastic uptrend changes in a bunch of different systems faults counts through a certain number of weeks. My data looks like this: And I would like to know what would be the ...
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Experiment design useful when objects change / will be replaced in each trial run?

I want to perform some studies on how material thickness, type of material, humidity and temperature influence tensile strength. While I was thinking about useful designs I was wondering whether a ...
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Average Sample Size For Curtailed Double Sampling Plan

I am currently implementing double sampling. The goal is to accept or reject a lot by only sampling a few instances from it. In a two-stage testing procedure (double sampling) one first draws a ...
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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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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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What is the rationale of using 2.66 times moving range for control limits in individual control charts?

The Wikipedia says "If the data are normally distributed with standard deviation $\sigma$ then the expected value of $\overline {MR}$ is $d_{{2}}\sigma =2\sigma /{\sqrt \pi }$." https://en....
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How many values should one validate to achieve a desired confidence?

If I have a set of 100 values which have been programmatically calculated and I would like to validate those calculations by performing them manually, how many manual calculations would I need to do ...
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determining the sample size for quality test

It has been a long time since I did statistics in uni and I have a "simple problem" that I need solving. I need to determin for a sample size for a limited population of documents ( lets say ...
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What are the suitable methods for statistical process control with non normally distributed data such as proportion?

Is there anyone familiar with statistical process control charts? The usual control limits +-3SD are based on normal distribution. What are the common methods for dealing with non-normally distributed ...
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What are the best-practices for validating phone records? [closed]

I have a bunch of telephone data including information on call start time, end time, and duration. I am trying to evaluate the quality of the dataset to determine if the phone call data are legitimate ...
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How to determine the lifetime?

I want to determine the average expected lifetime of devices of which I have data. This data comprises ~4500 generators of which ~3800 died and the others are still alive. Furthermore, I have ...
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Using standard deviation to calculate Control Limits for Individual Control Chart

To compute the control limits using the 3 sigma rule, the standard deviation is usually approximated by the formula: $\sigma=\frac{\overline{MR}}{1.128}$, where $\overline{MR}$ is the average of all ...
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Can I use an I-MR chart for a process in which the sampling time is random, or not regular?

I am currently studying a process which produces one sample for testing at one time. Not only that, the time at which the sample is taken for inspection is also not fixed but it varies. The process ...
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process tolerance limits - use data from apples to achieve compliance with upper limits on oranges; assuming implicit proportionality apples/oranges

What if there is a process where samples are tested two ways - apples and oranges, but you want to measure apples periodically as a way to achieve the tolerance limit on oranges. But there is a ...
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Is it better to use the prediction interval or the RMSE?

I use a measurement technique $X$ to assess if a good is within production specifications during its manufacturing. $X$ is slow but precise. I wish to find out if I can use a measurement technique $Y$ ...
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Models/techniques for financial data quality

I'm part of a team tasked with assuring data quality of a large database of financial data (credit cards, collaterals, etc..). We're essentially looking for anomalies, such as single outliers or ...
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three sigma vs six sigma

I wonder about the meaning of the term six sigma. It describes a process/production with (mostly) zero defects. You can see here: source: https://www.dummies.com/article/business-careers-money/...
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How to estimate probable response times, from previous samples?

I'm a IT manager that deals with delays from various departments in a purchase process. In a given phase we have 25 handovers and thus 25 response times. So many variable times (and without SLA) ...
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Balancing a sample for outsized response?

I recently ran a survey of 1,001 respondents online. 57.34% of the respondents were Millennials, 29.47% were Gen X, and 13.19% were Boomers. The topic was real estate, and I asked how many had bought ...
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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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Sample size and type of hypothesis test for QC of products (frozen cattle embryos)

I have a question regarding sample size and type of hypothesis test to be used. I am currently working on buying beef cattle embryos from overseas. The livestock department in my country is going to ...
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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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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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statistical process control for non normal time series

Let us say I have several KPI measurements per month. There may be different numbers of measurements per month but there are more than 30. The monthly distributions do not look normally as shown in ...
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XmR charts for several measures at the same day/month

Xmr charts are often used in statistical process control (see for example). They make sense, but I wonder what one does, if one has several measure per day/month etc. Should one just use the average (...
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Setting limits for process control of low probability error

I have a process which produces plastic parts. I have a requirement that states that the weight shall be > x grams. So far I have measured each individual part produced (around 250 pieces) and ...
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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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Using an old trained model on a new data - comparing two populations

I have two populations with the same set of features. The range of the features' values is also the same. I trained Random Forest Algorithm on the first population. Thus, I wonder whether or not I can ...
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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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 ...