How do you choose the right type of control chart for your process?

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Control charts are a powerful tool for monitoring and improving your process performance in lean operations. They help you detect and analyze variation, identify trends and outliers, and evaluate the effectiveness of your improvement actions. But how do you choose the right type of control chart for your process? In this article, we will explain the main types of control charts, their advantages and disadvantages, and how to use them in different situations.

本文章的要点总结

Data type determination:

Start by identifying whether your process outputs variable or attribute data. This choice will guide you to the most suitable control chart, streamlining your analysis.
Match tools to function:

Consider the specific area of your business when selecting statistical tools. For instance, Quality Assurance (QA) may benefit more from P or C-charts to pinpoint systemic issues.

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Daniel Holanda ??

??LinkedIn Top Voice??|Lean…

1 Variable data charts

Variable data charts are used for processes that produce continuous or measurable outputs, such as length, weight, time, or temperature. They show the distribution of the data around a central line, which represents the average or target value. The most common variable data charts are the X-bar and R chart, which plots the mean and range of subgroups of data, and the X and S chart, which plots the individual values and standard deviation of subgroups of data. Variable data charts are useful for detecting shifts or changes in the process mean or variation, and for comparing the process performance to specifications or customer requirements.

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Daniel Holanda ??

??LinkedIn Top Voice??|Lean Manufacturing |Industrial Engineering |Projects Leadership |Operations Management |Operational Excellence - SR. Business Excellence Coordinator |Engineering Supervisor |Operations Manager
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A escolha dos tipos de gráficos a se utilizar depende das necessidades específicas do processo em quest?o. Se a variabilidade do processo for alta e também para se ter uma vis?o mais completa, muitas vezes é útil usar mais de um tipo de gráfico de controle em conjunto ou até mesmo ferramentas mais completas de estatistica ou six sigma.

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Willya Marcolino

?? Top Lean Manufacturing Voice. Especialista em Excelência Operacional e Indústria 4.0, líder em Projetos de Redu??o de Custos e Gest?o de Equipes.
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Usar gráficos de controle X-barra e R ou X-barra e S em processos com alta variabilidade exige ajustes nos limites de controle para melhor detectar causas especiais de varia??o, ocultadas pelas flutua??es naturais. Esses gráficos podem necessitar ser complementados com outras ferramentas estatísticas e de qualidade como Six Sigma, especialmente em ambientes onde a variabilidade é alta.

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2 Attribute data charts

Attribute data charts are used for processes that produce discrete or countable outputs, such as defects, errors, or complaints. They show the frequency or proportion of the data that falls into certain categories or attributes. The most common attribute data charts are the P chart, which plots the fraction of defective items in a sample, the NP chart, which plots the number of defective items in a sample, the C chart, which plots the number of defects per unit, and the U chart, which plots the number of defects per unit of measure. Attribute data charts are useful for monitoring the process capability or quality, and for identifying the sources or causes of defects.

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Willya Marcolino

?? Top Lean Manufacturing Voice. Especialista em Excelência Operacional e Indústria 4.0, líder em Projetos de Redu??o de Custos e Gest?o de Equipes.
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Gráficos de dados de atributo s?o essenciais para impulsionar melhorias de qualidade em processos de produ??o ao permitir a identifica??o clara de tendências e padr?es de defeitos. Eles facilitam a análise de causa raiz, ajudando as equipes a descobrir e corrigir as origens dos problemas. Após implementar solu??es, esses gráficos também s?o úteis para monitorar a eficácia das interven??es, visualizando redu??es nos defeitos ao longo do tempo e validando melhorias.

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3 Specialized charts

Specialized charts are used for processes that have specific characteristics or requirements that are not well suited for the standard variable or attribute data charts. For example, some processes may have non-normal distributions, autocorrelated data, rare events, or multiple factors affecting the output. Some of the specialized charts are the EWMA chart, which uses an exponentially weighted moving average to smooth out random variation, the CUSUM chart, which uses a cumulative sum to detect small shifts in the process mean, the G chart, which plots the time between defects or failures, and the MEWMA chart, which plots the multivariate exponentially weighted moving average for processes with multiple correlated outputs. Specialized charts are useful for enhancing the sensitivity or accuracy of the control chart analysis, and for addressing complex or challenging process situations.

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Willya Marcolino

?? Top Lean Manufacturing Voice. Especialista em Excelência Operacional e Indústria 4.0, líder em Projetos de Redu??o de Custos e Gest?o de Equipes.
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A sele??o do gráfico depende da sensibilidade necessária para capturar mudan?as, do tipo de dados e da frequência dos eventos do processo.

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4 How to choose

When selecting the right type of control chart for your process, there is no one-size-fits-all solution. However, you can start by determining the type of data you have - variable or attribute - as this will narrow down your options. Additionally, consider the sample size and frequency of data collection, as this will affect subgrouping and plotting, as well as the calculation of control limits and statistics. Furthermore, define your objective and criteria for detecting and interpreting variation to select a chart that best fits your process performance indicators and improvement goals. Also, evaluate the assumptions and limitations of each chart to avoid potential errors or biases in your analysis. Lastly, experiment with different charts to find the best fit for your process and validate and refine your results.

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5 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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How do you choose the right type of control chart for your process?

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1 Variable data charts

2 Attribute data charts

3 Specialized charts

4 How to choose

5 Here’s what else to consider

Lean Operations

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