Textbook of Usability

Also known as: MHP

The Model Human Processor (MHP) is an engineering approximation of human cognition developed by Stuart Card, Thomas Moran, and Allen Newell in their landmark 1983 book The Psychology of Human-Computer Interaction. It treats the human as an information processing system with three interacting subsystems, each with its own processor and memory.

The three subsystems and their cycle times:

• Perceptual processor: ~100 ms (range 50–200 ms) — time to process one "frame" of input
• Cognitive processor: ~70 ms (range 25–170 ms) — time for a single cognitive operation
• Motor processor: ~70 ms (range 30–100 ms) — time to initiate and execute one motor action

The MHP also specifies memory parameters: working memory capacity of ~3 chunks with decay time of ~7 seconds per chunk, and long-term memory with effectively unlimited capacity.

The MHP can generate quantitative predictions by tracing information flow through the three subsystems. Simple reaction time ≈ τp + τc + τm ≈ 240 ms. Choice reaction time follows Hick's Law derived from the cognitive cycle: $T = \tau_c \log_2(n+1)$.

The MHP is deliberately simplified — it does not model emotion, fatigue, individual differences, or error. Its power is that it provides approximate quantitative predictions without human subjects testing, allowing early design comparisons before any prototype is built.