Functional Safety 2014

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November 2014

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Fig. 3 Plato’s Allegory of the Cave

A human’s brain has a tendency to strive to process information quickly without rationally

understanding it; consequently it omits detail which results in mistakes and errors.

The following is an example that shows that a human’s brain is capable of accurately constructing

words without the words being written correctly. Below is a paragraph written by G.E. Rawlinson

that most people have the capability of reading accurately despite the letters being jumbled up

“Aoccdrnig to rscheearch at Cambridge Uinervtisy, it deosn't matter in what order the letters

in a word are, the only iprmoetnt thing is that the first and lsat ltteer be at the rghit pclae. The

rset can be a toatl mses and you can sitll raed it wouthit porbelm. Tihs is bcuseae the huamn

mnid deos not raed ervey lteter by istlef, but the wrod as a wlohe.”

The above shows that human brain has the capability to interpret how it wants to read rather than

reading it literally, i.e. it assumes corrections to the passage which may or may not align with what

the author intended; this demonstrates all three synopsis of Plato’s cave.

When may systematic failures be introduced?

ISA-TR84.00.02-2002 Part 2 states “Systematic failures may be introduced during the specification,

design, implementation, operational andmodification phase and affect hardware as well as software,”

i.e. at any stage in the safety lifecycle

The ISA standard shows the inclusion of both random hardware failures and systematic failures in SIL

verification calculations for the average probability of dangerous failures on demand (PFDavg). It

G.E. Rawlinson, “The significance of letter position in word recognition” 1976