One should avoid carrying out ...

One should avoid carrying out an experiment requiring more than 10 per cent accuracy.
 Walther Nernst

Quotes from the same author

On examinations: Das Wissen ist der Tad der Forschung. Knowledge is the death of research. Nernst's motto.
 Walther Nernst
No effect that requires more than 10 percent accuracy in measurement is worth investigating.
 Walther Nernst