  Measuring the deviation from scale invariance of the primordial power
spectrum is a critical test of inflation. In this paper we reconstruct the
shape of the primordial power spectrum of curvature perturbations from the
cosmic microwave background data, including the 7-year Wilkinson Microwave
Anisotropy Probe data and the Atacama Cosmology Telescope 148 GHz data, by
using a binning method of a cubic spline interpolation in log-log space. We
find that the power-law spectrum is preferred by the data and that the
Harrison-Zel'dovich spectrum is disfavored at 95% confidence level. These
conclusions hold with and without allowing for tensor modes, however the
simpler model without tensors is preferred by the data. We do not find evidence
for a feature in the primordial power spectrum - in full agreement with generic
predictions from cosmological inflation.
