This topic introduces three foundational pillars of computer system design: Usability, Security, and Accessibility. Understanding how these concepts interact is essential for evaluating and improving human interaction with technology.
Usability is the measure of how easily and efficiently users can interact with a device or system to achieve their goals. A usable system is effective, efficient, and satisfying to use.
| Component | Description |
|---|---|
| Learnability | How easy is it for users to accomplish basic tasks the first time? |
| Efficiency | Once learned, how quickly can users perform tasks? |
| Memorability | When users return after a period of not using the system, how easily can they re-establish proficiency? |
| Errors | How many errors do users make, how severe are they, and how easily can they recover? |
| Satisfaction | How pleasant is it to use the design? |
Security in computing refers to protecting systems, networks, and data from unauthorized access, damage, or attack.
The three core principles of information security are:
| Principle | Meaning | Example Threat |
|---|---|---|
| Confidentiality | Keeping data private and accessible only to authorized users | Data breach, eavesdropping |
| Integrity | Ensuring data is accurate and has not been tampered with | Man-in-the-middle attack, data corruption |
| Availability | Ensuring systems and data are accessible when needed | Denial of Service (DoS) attack |
Key insight: A DoS attack targets Availability — it does not steal data but makes the system unreachable for legitimate users.
There is a well-known Security-Usability Trade-off: as security measures become stricter (e.g., longer passwords, frequent re-authentication, CAPTCHAs), the system becomes harder and more tedious to use. Designers must balance both.
Accessibility is the design of devices, systems, and services so that they can be used by people with a wide range of abilities and disabilities — including visual, auditory, motor, and cognitive impairments.
The Web Content Accessibility Guidelines (WCAG) define four principles, remembered as POUR:
| Principle | Meaning | Example |
|---|---|---|
| Perceivable | Information must be presentable in ways users can perceive (sight, sound) | Alt text for images, captions for videos |
| Operable | All functionality must be operable via various inputs | Full keyboard navigation (no mouse required) |
| Understandable | Content and operation must be understandable | Clear language, consistent navigation |
| Robust | Content must be interpreted reliably by assistive technologies | Compatibility with screen readers |
These three concepts are deeply interconnected:
Effective system design requires balancing all three.