Cardiel López, NicolásGorgas García, Francisco JavierCenarro, J.González, J. J.2023-06-202023-06-201998-020365-0138https://hdl.handle.net/20.500.14352/59537We are grateful to the referee for useful comments. The JKT is operated on the island of La Palma by the Royal Greenwich Observatory at the Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. The Calar Alto Observatory is operated jointly by the Max-Planck-Institute für Astronomie, Heidelberg, and the Spanish Comisión Nacional de Astronomía. This work was supported by the Spanish “Programa Sectorial de Promoción General del Conocimiento” under grant No. PB93-0456.We present a new set of accurate formulae for the computation of random errors in the measurement of atomic and molecular line-strength indices. The new expressions are in excellent agreement with numerical simulations. We have found that, in some cases, the use of approximated equations can give misleading line-strength index errors. It is important to note that accurate errors can only be achieved after a full control of the error propagation throughout the data reduction with a parallel processing of data and error images. Finally, simple recipes for the estimation of the required signal-to-noise ratio to achieve a fixed index error are presented.engjournal articlehttp://arxiv.org/pdf/astro-ph/9706116v1.pdfopen access52Old stellar populationsEarly-type galaxiesExtragalactic globular-clustersEarly-type systemsElliptical galaxiesSpectral evolutionK-giantsGradientsSpectroscopyIndexesAstrofísicaAstronomía (Física)