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

The SPC (Statistical Process Control) Charts processor analyzes a window of process data and calculates statistical measures that assess a process's performance and capability against its specification limits, using control charts and predefined rules.

## Why use SPC Charts

Use this processor to monitor whether a process stays within its specification limits and to catch early warning patterns using Nelson Rules, rather than only watching a raw value against fixed thresholds.

## How it works

The processor calculates a current mean and standard deviation (`stdDev`) from the input data within the window, then derives control limits and capability indices from them:

- Upper Specification Limit (USL) and Lower Specification Limit (LSL) are calculated from the current mean and standard deviation.
- Process capability (`Cp`) is the ratio of the difference between USL and LSL to six times the standard deviation (6σ).
- `CPu` (Upper Capability Index) and `CPl` (Lower Capability Index) measure how centered the process is within the specification limits: `CPu = (USL - average) / 3σ`, and `CPl = (average - LSL) / 3σ`.
- Process performance (`Cpk`) is the smaller of `CPu` and `CPl`.
- If valid data is present, the processor also calculates `Cpm` and `Cpkm`, which incorporate **Target Mean**.

If your input tag doesn't publish at a steady rate, use **Time Interval** to publish a value on a fixed schedule instead of waiting for the next incoming event. Enter `0` to disable the timer if your input already publishes at the expected rate.

### Nelson Rule violations

The processor flags a `true` value for each Nelson Rule your data violates:

- One point is more than three standard deviations from the mean.
- Nine or more points in a row fall on the same side of the mean.
- Six or more points in a row continually increase or decrease.
- 14 or more points in a row alternate direction, increasing then decreasing.
- Two out of three, or three out of three, points in a row fall more than two standard deviations from the mean in the same direction.
- Four out of five, or five out of five, points in a row fall more than one standard deviation from the mean in the same direction.
- 15 points in a row all fall within one standard deviation of the mean, on either side.
- Eight points in a row occur, with none within one standard deviation of the mean, and the points fall in both directions from the mean.

## Parameters

| **Parameter**      | **Details**                                                                                                                                                                       |
| ------------------ | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| Window Size        | The number of data points considered for calculations. Affects the current mean, standard deviation, and specification limits. Defaults to `50`.                                  |
| Target Mean        | The desired average value for the process, used as a reference point for evaluating performance. Required, along with valid data, to calculate `Cpm` and Cpkm. Defaults to `100`. |
| Pass Through Value | Determines whether the processor's output is the calculated control chart fields or the original unchanged input value.                                                           |
| Time Interval      | Sets a timer that publishes output on a fixed schedule if the input tag isn't polling regularly. Enter `0` to disable the timer.                                                  |

## Related topics

- [Use the SPC Charts Function](docId\:h7fkYhWw8XueZtpNOdsa6)