# 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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### 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 ...
32 views

### Statistical Approaches for Interpreting Weekly Changes in a Percentage Metric

Overview Each week we have some number of trials and successes, and we track the percentage of successes each week, this is our metric. Made up example: A loan company - each week we get some ...
297 views

### The link between AQL, RQL and the reliability of a process

The problem is to distinguish between the definitions of AQL, RQL and reliability in the field of quality control. Let us just pose the context. We wish to calculate the reliability of a manufacturing ...
19 views

### Different methods to calculate standard deviation

I am doing a study using Statistical Process Control and have to calculate the standard deviation. I am aware of a number of ways to do this. The first method is using the sum of squares (classic ...
490 views

### 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. ...
9 views

### Linear Model + tolerance intervals: How to show that when a condition is fulfilled for a tool it is also fulfilled for another related tool?

The problem Imagine that you have two different tools that produce data at given time points $t_1,...,t_m$. Suppose that we only have access to the mean and standard deviation of the data at each time ...
32 views

### Statistic for assessing quality of dose response curve (fit with logistic regression)

I want to assess the quality of a 4-parameter logistic regression fit on dose response data. I am in the process of assessing the quality of some compound screening data. This is characterising the ...
14 views

### Equivalence of two processes or implication that one is better than the other

The problem Two different tools $M1$ and $M2$ are supposed to deliver a certain amount of volume of a liquid but in different ways. Furthermore, the efficacity of the tools to deliver this volume ...
82 views

### Probability of detecting a shift in fraction nonconforming control chart

I try to solve a problem (number 7.7) in the 6-th edition of the book 'Statistical Quality Control, a modern introduction' of Douglas Montgomery. Here is the statement: A control chart indicates that ...
47 views

### 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 ...
15 views

### Questions about Using Monte Carlo simulation to determine the control limits of control chart

The Montacaro simulation method is used to solve the control limit of the control chart, how to determine the randomly generated data streams, and whether it needs to be generated according to the ...
2k views

### 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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### 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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### Data Quality check in time series data

I have time series data. The table contains date, no of orders, no of products sold, no of returns, no of walk in customers, etc. I want to create a model which takes 12 months of data as input and ...
1 vote
189 views

### Difference between anomaly detection and data quality check

I am a beginner in ML and in my company i have been asked to come up with models that can check if there are data quality issue in any given table. It will be an unsupervised learning task and I only ...
1 vote
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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 ...
16 views

### Statistical Quality Control methods for data quality

I had a minor course in Statistical Quality Control (SQC) for manufacturing industries. Here I learnt about Exponential Weighted Moving average Charts, X-bar R charts and CUSUM charts to name a few. ...
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### How to apply Bayesian statistics to this (general) Quality Control problem

This more an academic question than practical. I like that ;) I have been thoroughly trained in frequentist statistics. Zealously, actually - to the point that no one even told me it’s called “...
100 views

### What does the quality of representation of a variable mean in PCA?

I understand that the quality of representation of an individual by a certain axis is measured by the cosine of the angle between the axis and the individual; the more the vector representing the ...
1 vote
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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 ...
36 views

### Control Chart: Nelson alternating rule

The Nelson rules for control charts describe patterns, which are "special" and need our attention. One of these rules is the "alternation rule". According to Nelson it is "...
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### What is the proper test to determine if a result is above the normal set of values?

I'm a chemist who has been asked to put my statistics hat on, and I'm looking for some help. Situation: Every month we get a report with the number of guest complaints for each item we sell. Each ...
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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 ...
1 vote
42 views

### Repeats vs Replications

I've seen e.g. the minitab explanation on this topic which isn't bad but though, in the end, I can't explain which statistical term or the like I would (or could) investigate with each of them. I ...
25 views

### 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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### 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 ...
162 views

### 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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### 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) ...
694 views

### 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 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 ...
1 vote
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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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### Performance testing: detecting changes in distributions over time

I have an interesting application I’d like some advice on. A common task when working with stochastic systems in an engineering context is testing for regressions and improvements in performance. ...
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### Find significant change in sensor data

I have a sensor, which produces normal distributed data under usual conditions. If there is some defect the values will increase for a certain timespan. I want to detect such a change using a ...
1 vote
148 views

### 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 ...
658 views

### 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....
51 views

### 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 ...
66 views

### 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 ...
146 views

### 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 $Y$ ...
1 vote
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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/...
179 views

### 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 ...
1 vote
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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 ...
2k views

### 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 ...
11k views

### How find controls limits for CUSUM chart

I am trying to re-do a simulation work in quality control. In this simulation they use bisection search algorithm to find the control limits h (the threshold) that gives the desire average run length ...
1 vote