# Limitations of human mind

Why do we care about them?

• To understand why artificial systems are the way they are, including complexity
• To understand the sources of errors, bugs, and disasters
• To guide design of our systems
• To guide evaluation of our systems

## Manifests of a limited human mind

### How fast can we make a decision?

The Hick-Hyman Law of decision making: T=a*log(n+1) for n equally possible choices. The logarithm suggests a tree representation of choices when we evaluate them. +1 because of uncertainty about whether to respond

T=a*pi*log(1/pi+1)

#### How fast can we move our hand?

Fitts' Law: T=a+b*log(1+D/W)

One can derive Steering Law from Fitts' Law

### Rate of Communication with machines and humans

#### How fast can we type?

Words per minute: 33 wpm for transcription by average computer users; 50 to 80 for professional typists

#### How fast can we read?

college students can raud (read with comprehension) at a typical rate of 300 wpm. (Source: R.P. Carver, Reading Rate: A Review of Research and Theory. Academic Press, 1990.)

#### How fast can we speak?

Audiobooks: 150-160 wpm.

How many words can we speak in a day? This leads to a fundamental limit to our communication bandwidth with peers and the size of an effective team.

When adding staff to a project, ask what exactly is the bottleneck: communication or dirty, textbook engineering. Quote from MMM (The Mythical Man-Month, book recommended for the course): Brooks's Law ``Adding manpower to a late software project makes it later''

Organization principles to reduce the number of inter-person communication links: hierarchy, layering

To email or to meet:If you need to communicate a message of W words to N people, should you write it up as an email or call a meeting? Assume people's time is equally valuable and your goal is to deliver the message with the least total cost (measured by total time spent by all people including yourself).

## Inner works of human mind

### Memory

Why memory is important

• Memory is our identity: how are you sure you are the physical continuation of yourself when you wake up? Read about the Swampman thought experiment
• Memory is our intelligence
• Short-term memory is the working foundation for reasoning and the conscious process (System 2 in the word of Daniel Kahneman).
• Long-term memory is the working foundation for the subconscious process (System 1) and our expertise.

#### Short-term memory

• Fast and operational
• Small:~7 entries

Implications

• Human cognition is serial at the symbolic level (Despite its neural implementation is parallel)
• Hierarchy and modularization in artificial objects
• Language: syllables, word, sentence, paragraph (How long is each?). Recall Flesch-Kincaid readability
• Writing
• Computer

#### Long-term memory

• Slow, not operational
• ~8 seconds to transfer a chunk to long-term memory
• Unlimited size
• Representation matters
• Synthetic: (1) remember the gist; (2) fill in the details; and (3) believe them

Expertise is really about long-term memory

• How many chunks of information
• Learning ~50,000 chunks of information; 10 years of experience (Herbert Simon)
• How they are represented

Serial position effect: Recency bias vs. Primacy vias