---
title: SPC Charts
slug: litmusedge-v2/spc-charts
docTags: 
createdAt: 2024-10-21T18:05:57.686Z
---

The **SPC Chart **processor function helps analyze process data over a specified window and generate insights into the process performance and capability. It calculates various statistical measures and assesses adherence to quality specifications using control charts and pre-defined rules.

# SPC Charts Overview

- First, the **target\_mean** (default 100), **window size** (default 50), and **timer interval** need to be set.
- The **timer interval** parameter ensures that even if the connected input tag is not polling, the SPC chart will still publish data at the specified interval.
  - If the input is expected to publish at the expected rate, set the timer interval to **0** to disable the timer.
- The **current mean** and **standard deviation (stdDev)** are calculated based on the input data within the defined window.
- **Upper Specification Limit (USL)** and **Lower Specification Limit (LSL)** are computed based on the **current\_mean** and **standard deviation**.
- The **process capability (Cp)** is calculated as the ratio of the difference between USL and LSL over six times the standard deviation (6σ).
- **CPu** (Upper Capability Index) and **CPl** (Lower Capability Index) are calculated to assess how centered the process is within the specification limits.
- CPu = (USL - avg) / 3σ, CPl = (avg - LSL) / 3σ
- **Process performance (Cpk)** is calculated by determining the smallest value between CPu and CPl.
- Optionally, if valid data is present, the **Cpm** and **Cpkm** (which use the target mean) will be computed.
- The chart will also output any **Nelson Rule violations** with a **true** flag indicating which rule number has been violated.
- The rules are as follows
- Rule1: One point is more than 3 standard deviations away from the mean.
- Rule2: Nine (or more) points in a row are on the same side of the mean.
- Rule3: Six (or more) points in a row are continually increasing or decreasing.
- Rule4: Fourteen (or more) points in a row alternate in direction, increasing then decreasing.
- Rule5: Two (or three) out of three points in a row are more than 2 standard deviations away from the mean in the same direction.
- Rule6: Four (or five) out of five points in a row are more than 1 standard deviation away from the mean in the same direction.
- Rule7: Fifteen points in a row are all within 1 standard deviation of the mean on either side of the mean.
- Rule8: Eight points in a row exist, but none within 1 standard deviation of the mean, and the points are in both directions from the mean.

# SPC Charts Parameters

| **Parameters** | **Details**                                                                                                                                                                              |
| -------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| Window Size    | 
Defines the number of data points considered for calculations. The window size affects the computation of the current mean, standard deviation, and specification limits.

 |
| Target Mean    | This represents the desired average value for the process. It serves as a reference point for evaluating process performance against expectations.                                       |
| Time Interval  | Specifies the frequency at which the SPC chart publishes data.                                                                                                                           |

![](https://api.archbee.com/api/optimize/SSUUxKZUk9bFTEPNn_6Zo/GTO9WkoMPYQLB41P4Iwph_image.png)

:::hint{type="info"}
**Note: **When creating an analytics flow with *SPC Charts *processor, refer the [Use the SPC Charts Function](docId:_5Zm_3cvddVcXPw90uoyr) guide for more details.
:::