  We compare three independent, cosmological linear perturbation theory codes
to asses the level of agreement between them and to improve upon it by
investigating the sources of discrepancy. By eliminating the major sources of
numerical instability the final level of agreement between the codes was
improved by an order of magnitude. The relative error is now below 0.1% for the
dark matter power spectrum. For the cosmic microwave background anisotropies
the agreement is below the sampling variance up to l=3000, with close to 0.1%
accuracy reached over most of this range of scales. The same level of agreement
is also achieved for the polarization spectrum and the temperature-polarization
cross-spectrum. Linear perturbation theory codes are thus well prepared for the
present and upcoming high precision cosmological observations.
