---
title: "Control charts"
subject: "ICT Project Management"
chapter: 10
key: "semester_7/ict_project_management:control-charts"
source: https://www.bctengineer.com/q/semester_7/ict_project_management/control-charts
site: BCT Engineering
---

# Control charts

ICT Project Management · Chapter 10: Project quality management

- Asked in: year not given
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/ict_project_management/control-charts
- Whole chapter: https://www.bctengineer.com/md/semester_7/ict_project_management/chapter-10

## Question

What do you mean by control charts?

## Short answer

A control chart plots process data over time with a mean and ±3σ control limits to distinguish common cause variation from special causes. A point outside limits or seven consecutive points on one side signals an out-of-control process needing investigation.

## Answer

A **control chart** is a graph used to study how a process changes over time. Data are plotted in time order, with a **central line** (average) and upper and lower **control limits** (usually ±3 standard deviations) that show the expected variation of a stable process. It is one of the seven basic quality tools, developed by Walter Shewhart.

#### Parts
```text
Value
                              * (out of control)
UCL ---------------------------------  upper control limit
      *       *
 CL --*---*-*---*---*---*--*---------  centre line (mean)
         *       *
LCL ---------------------------------  lower control limit
      1 2 3 4 5 6 7 8 9 10 ...  sample / time
```

#### Purpose
- Determine whether a process is **in control** (stable, with only random/common causes) or **out of control** (special causes present).
- Monitor the stability of project processes, such as defects per build, response time or schedule variance.
- Prevent defects by detecting trends early.
- Decide whether corrective action is needed.

#### Interpretation
- A point outside the control limits means an assignable cause that needs investigation.
- **Rule of seven**: seven consecutive points on one side of the mean show a non-random pattern (trend) even if they are within limits.
- Trends or cycles indicate a problem.
- Control limits differ from **specification limits** (the customer's requirement). A process may be in control but still not meet specifications.

#### Types
| Type | Data |
|---|---|
| X-bar and R chart | Variable data (measurements) |
| p-chart | Proportion defective |
| c-chart / u-chart | Number of defects per unit |

#### Steps to prepare
1. Select the characteristic to measure.
2. Collect samples at regular intervals.
3. Calculate the mean and standard deviation; set UCL = mean + 3σ, LCL = mean − 3σ.
4. Plot and interpret.

**Example:** A team plots the number of bugs found per release. A point above the UCL after a new library was introduced shows a special cause, which leads to a review of the library.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How are control limits usually set?
   A. Mean plus and minus 3 standard deviations
   B. Mean plus and minus 1 standard deviation
   C. Specification limits
   D. Maximum and minimum values
   Answer: A. UCL = mean + 3 sigma, LCL = mean - 3 sigma. (Key point: Limits)

2. What does a point outside the control limits indicate?
   A. Normal random variation
   B. A special (assignable) cause needing investigation
   C. A perfect process
   D. The customer specification is met
   Answer: B. Points beyond limits signal special causes. (Key point: Out of control)

3. What does the rule of seven say?
   A. Seven points must be above UCL
   B. Seven samples are required
   C. Seven consecutive points on one side of the mean indicate a non-random pattern
   D. Seven charts must be drawn
   Answer: C. Even within limits, seven in a row on one side shows a trend. (Key point: Rule of seven)

4. How do control limits differ from specification limits?
   A. They are the same
   B. Control limits are set by the customer
   C. Specification limits come from standard deviation
   D. Control limits reflect process variation; specification limits are customer requirements
   Answer: D. A process may be in control yet fail specifications. (Key point: Control vs specification)

## Related in ICT Project Management

- [Pareto charts](https://www.bctengineer.com/question.md?key=semester_7%2Fict_project_management%3Apareto-charts) — ICT Project Management, chapter 10
- [Integrated change control](https://www.bctengineer.com/question.md?key=semester_7%2Fict_project_management%3Aintegrated-change-control) — ICT Project Management, chapter 7
- [Project scope control](https://www.bctengineer.com/question.md?key=semester_7%2Fict_project_management%3Aproject-scope-control) — ICT Project Management, chapter 7
- [Schedule development and control](https://www.bctengineer.com/question.md?key=semester_7%2Fict_project_management%3Aschedule-development-control) — ICT Project Management, chapter 8
- [Test life cycle in quality control](https://www.bctengineer.com/question.md?key=semester_7%2Fict_project_management%3Atest-life-cycle) — ICT Project Management, chapter 10

## Related in other subjects

- [Constructing and interpreting X-bar and R charts](https://www.bctengineer.com/question.md?key=semester_3%2Fmetrology%3Ax-bar-r-chart-construction) — Metrology, chapter 15
- [Quality control of concrete at site](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aquality-control-at-site) — Concrete Technology and Masonry Structure, chapter 5, asked 11×
- [Case study: current control and recommendation](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Acs-existing-recommend-problems) — Instrumentation II, chapter 10, asked 3×
- [Project control and control cycle steps](https://www.bctengineer.com/question.md?key=semester_7%2Fproject_engineering%3Aproject-control-cycle-steps) — Project Engineering, chapter 4, asked 3×