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

Pareto chart focusing on the trivial many

Pareto chart is mostly focusing on the vital few. But I have heard that nowadays, some are focusing first on solving the trivial many, believing that it is easier to solve and may support to lessen ...
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59 views

Pareto Chart: Determining the 80% and 20% part

Which one is true? What part is really be 80% and 20%? Edit: Clarification and additional details to the problem. According to the pareto principle, it states that loosely 80% of the effects come from ...
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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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Control limits in Quality control

Agree or disagree with the below statement “A statistically controlled process will always produce hundred percent results within tolerance limits” My views : the control limits are taken as 3*s.d. ...
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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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Is there a standard for setting quality acceptance limits in manufacturing for continuous measurements?

In the resources I've found, acceptance sampling is posed in terms of taking a certain number of units from a batch and measuring how many are defective (NIST website: https://www.itl.nist.gov/div898/...
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16 views

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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good quotation on the control part of statistical quality control?

I'm writing a paper making use of techniques from statistical quality control, and would like to include a quotation that expresses a sentiment something like this: Statistical quality control is not ...
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107 views

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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45 views

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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20 views

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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How to handle missing points in statistical control charts?

If I have time-series data with 'n' number of points missing/absent from the data, is there a way or test to determine whether I have enough points to create and interpret a control chart based on the ...
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176 views

Offline Precision@k and Recall@k for recommender system

How can I evaluate offline Precision@k and Recall@k metrics for recommender system if I only have items-users matrix? I think I can't just compare recommendations and user data because it will be ...
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What is drift sensitivity and how it does it relate to SPC sampling optimization?

Drift sensitivity denoted by k where k = 3*(Cp-Cpk). I came across above while doing a review in SPC documentation in our company. However, it was not explained ...
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13 views

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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42 views

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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57 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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101 views

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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52 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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31 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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211 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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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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262 views

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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31 views

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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45 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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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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306 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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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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15 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
613 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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84 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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401 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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99 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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97 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
343 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
125 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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154 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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111 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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348 views

CUSUM test for regression model

I guess my question is rather basis. 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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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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244 views

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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1answer
56 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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2k 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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272 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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107 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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274 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 ...