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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How do I get people to take better care of data?
My workplace has employees from a very wide range of disciplines, so we generate data in lots of different forms. Consequently, each team has developed its own system for storing data. Some use ...
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Quality assurance and quality control (QA/QC) guidelines for a database
Background
I am overseeing the input of data from primary literature into a database. The data entry process is error prone, particularly because users must interpret experimental design, extract ...
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Why isn't bayesian statistics more popular for statistical process control?
My understanding of the bayesian vs frequentist debate is that frequentist statistics:
is (or claims to be) objective
or at least unbiased
so different researchers, using different assumptions can ...
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How to interpret a control chart containing a majority of zero values?
I am using a control chart to try to work on some infection data, and will raise an alert if the infection is considered "out of control".
Problems arrive when I come to a set of data where ...
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Problems with Outlier Detection
In a blog post Andrew Gelman writes:
Stepwise regression is one of these things, like outlier detection and
pie charts, which appear to be popular among non-statisticians but are
considered by ...
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How to verify extremely low error rates
I am faced with trying to demonstrate through testing an extremely low error rate for a sensor (no more than 1 error in 1,000,000 attempts). We have limited time to conduct the experiment so we ...
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Given a control chart that shows the mean and upper/lower control limits, how do I tell if the cause of out of control points is assignable or not?
I am given 15 points. The control limits are at +/- 3 $\sigma$. Points 1, 4, 5, 6, 7, 8, 9, 10, 11, 13, and 15 fall within the control limits. Points 2, 3, 12, and 14 are outside of the control limits,...
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Understanding statistical control charts
Control charts are all the rage at my company these days. Our "data analysts" (quotes are on purpose) are putting control limits on almost all graphs that they produce. The other day we were looking ...
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What is the mathematically rigorous definition of chunky data?
When in the workplace, certain measurement-taking devices are subject to different numerical accuracy; in some cases, the accuracy can be pretty weak (i.e., to one or two significant values only). ...
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"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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Probability of failure in a finite population
I regularly inspect finite populations for failures (we make custom products in batches of ~500-800). Currently, we inspect every product for failure, which is quite a bit of work. I want to reduce ...
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How to judge if a datapoint deviates substantially from the norm
This is Statistics 101, but I'm not a statistician and so can't seem to find the right technical jargon to google.
My company collects data at discrete points through time. Today's datapoint is ...
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Measuring k-means clustering quality on training and test sets
I'm working on implementing streaming k-means in Mahout. The code is mostly done and we're talking about how to integrate the code.
As part of the quality evaluations, I want to know how the ...
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Software for learning statistical quality control
I'm taking a course in Statistical Quality Control as an elective in my masters program. We're using Montgomery's Introduction to Statistical Quality Control, 6th Edition.
The book uses minitab ...
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Algorithim to determine if point is "too far from the average"
Long story short I have a collection of about 30 scripts that manipulate data sets and place them in a database. These scripts report their running times as well as any errors that occur to a ...
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How to calculate margin of error for a binomial quality control experiment where only successes are observed (including FPC)?
Wikipedia's Margin of Error entry says that
a random sample of size 400 will give a margin of error, at a 95%
confidence level, of 0.98/20 or 0.049 - just under 5%
given an infinite population ...
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Advice on a good production level SPC (statistical process control) package?
I am tasked with bringing up an SPC system within our moderately sized, but very technical, manufacturing company.
It might be against the rules of this forum, or maybe bad form, to name specific ...
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Dealing with zeros in a poisson regression
Our code goes through multiple stages of review. I wish to use the number of defects at an earlier stage of review as a "defect density" estimate for later stages.
It sometimes happens that code has ...
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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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Looking for aberrations in time based data
Looking at IO latency data for a storage array. When a drive is about to fail, one indication is an increase in the IO operations 'time to complete'. The array kindly provides this data in this format:...
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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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Best statistics to show outliers beyond 2 SD
I try to perform scheduling for an organization based on client flows. We are very seasonal. Our senior management in Washington, DC make us base our schedule on the mean number of clients we have ...
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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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Average run length
Suppose in an Xbar chart, k successive points plotted outside a control limit indicates an out of control situation. If p is the probability of plotting a point outside a control limit, how is ARL = $(...
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How to calculate the probability for pattern that violates "control chart rules" to occur?
I am going to host a training session to teach healthcare staff how to use control chart (c-chart to be specific), and I need to tell that why these rules (Western Electric Rules) are so called rules ...
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What do the letter and the subscript in the Shewhart's control chart constants ($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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When is a single data point enough to be considered a trend in the data?
In the general case, when collecting data points at disparate points in time (such as once per day or two), under what conditions can a single low data point be considered a downwards trend in the ...
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Meta analysis: How to assess the quality of each of the individual studies in a systematic way?
I have a meta analysis at hand. I have reported on the quality of the included studies in a narrative form. But after I was requested about "quality assessment". I guess it might be something else, a ...
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Computing unbiased estimators of $\sigma$ for m samples of size n
Three unbiased estimators of standard deviation have been presented in the literature for m samples of size n. The first one is based on the sample ranges and can be obtained by
$\hat \sigma_1 = \...
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Sample size and Quality Control
Why does quality control becomes difficult when the sample size is large?
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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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Closed formula for D4 constant calculation? (Moving range chart constant)
I need to build a Moving Range Shewhart control chart given a series of observations. In short, I have to calculate the central line and the upper and lower limits as follows
...
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Method to remove bad values in time series (bad values known to take on a particular value)
This sounds easy, but I don't know of a good statistical method for it.
I have a time series that has (good) data points that range from ~3.5 to 30. The data are collected by an automated sensor. ...
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How to properly set up hypothesis testing in an operations reporting environment
I am in charge of quality control of n = 400 reports. Most of the quality checks are automated, but some have to be manual. I need to figure out how many reports my team needs to check manually in ...
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Quality control - non-normal distribution
Suppose we have a production line, which makes widgets. Assume also that we know distribution of widget's length ~ $N(\mu, \sigma)$ We measure widget's length and if it is within $\mu+/- 2 \sigma$ we ...
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On properly setting up a control chart
Let me present to you the way I have being 'doing' control charts up until now.
I have a series of $k$ samples of size $n$. I calculate the the sample means $\bar{x}_i$ and the grand mean $\bar{\bar{...
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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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How can I determine if a time-series is statistically stable?
I have time-series data that tracks the number of sydromics records my organization receives each week. The number of records had been steadily increasing as more organizations started sending us data ...
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What is "the" correct way of calculating Cp/Cpk values for Statistical Process Control?
There seems to be many different methods of calculating $C_p$/$C_{pk}$ values in SPC, and I need help in determining which one to use at any given situation.
Currently, we need to show $C_p$/$C_{pk}$ ...
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Color standards for control charts
Are there any references or standards that dictate the color coding of the zones of a Shewhart style control chart. We use green in +/- 2 sigma range, yellow between 2 and 3 sigma and red beyond 3 ...
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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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How to test for deviation from a uniform distribution?
I have a random number generator as part of a system that - putatively - randomly selects one of 10,000 drive locations to store an incoming file fragment. Multiple file fragments can fill a drive ...
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Where can I find tests that validate the output of popular statistical software? (e.g. R, SPSS, SAS)
This has been difficult to search on Google. I've looked on R's website as well. I assume there are test suites that validate that R, SPSS, SAS, et al are giving the right output for a given analysis? ...
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How can we say that a clustering quality measure is good?
There are few well known measures like silhouette width (SW), the Davies- Bouldin index (DB), the Calinski-Harabasz index (CH), and the Dunn index .
How can we say that a clustering quality measure ...
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Control limits in $\bar{x}$ and R chart
I am learning a Quality Control course and in theory part I don't understand calculating the control limits for $\bar{x}$ and R chart.I want to know, say when I calculate the R chart control limits no ...
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Proofreading lots of documents based on small sample
I have gazillion documents (your tax return) that I need to check for correctness, but I don't the manpower nor the will power to read through all of it. Even if I do, I can not guarantee the quality ...
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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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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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Using control charts with very large subgroup size?
I am working with a very large data set -- time-series data from an on-line process monitor with a 10 second measurement interval. I am trying to develop control limits for the process using control ...
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