Weber-Fechner law
Not a bias
A law of perception stating that we notice changes in proportion to the starting amount. A small change stands out against a small amount but disappears against a large one, and this shapes how we judge money and numbers too.
In everyday life
In a quiet library you notice a whisper, but in a loud restaurant someone has to raise their voice a lot before you notice any change.
You would drive ten minutes to save $15 on a $30 pair of headphones, but not to save the same $15 on a $900 sofa.
Why your mind does this
Our senses and judgment work by comparison. Noticing differences from a starting point, or from what came just before, is faster and cheaper than measuring things in absolute terms, and changes are often what signal danger or opportunity.
Its family, in 30 seconds: Change
How to spot it
Ask yourself whether you are reacting to the actual size of a change or only to its size compared to the total.
What you can do
When money is involved, look at the savings or cost as a plain amount and ask whether that sum would matter to you on its own.
Same family: Change
SourcesDehaene (2003)·Wikipedia