138 Uncertainty Principle

Measuring things can change them, often making the results and subsequent measurements invalid.

• Measuring sensitive variables in a system can alter them, undermining the validity of the results and the instrument of measure.

• For example, event logging in computers increases the visibility of how the computer is performing, but it also consumes computing resources, which interferes with the performance being measured.

• The uncertainty introduced by a measure is a function of the sensitivity of variables in a system and the invasiveness of the measure. Beware using invasive measures, for they can permanently alter system behaviors and lead to unintended consequences.

• Use minimally invasive measures of performance whenever possible. Remember the maxim: Not everything that can be counted counts, and not everything that counts can be counted.

See Also Abbe • Feedback Loop • Garbage In-Garbage out

Many weight-loss programs promote keeping a food journal to lose weight—i.e., writing down everything you eat. Why does this work? Tracking what you eat changes how you eat.