Noise and significance

Is that a real change.

Everything fluctuates

Metrics move day to day without any cause worth investigating. Reacting to every wobble produces explanations for randomness and a great deal of wasted work.

Look at the historical range of variation before calling a change unusual.
Small denominators produce large percentage swings by arithmetic.

Control charts

Plotting a metric with its normal range makes ordinary variation visible and unusual movement obvious. It is a simple defence against overreacting.