Comparison of Spectral Index Determinations
  The index $n$ of a power law power spectrum of primordial density fluctuations, $P(k) \propto k^n$, has been estimated using many different techniques. The most precise compare the COBE DMR large angular scale $\Delta T$ to the amplitude of the large scale structure, but these are also the most model-dependent. The COBE DMR $\Delta T$ has also been compared to the degree-scale $\Delta T$ from several experiments. And finally, a relatively model-independent value of $n$ can derived from the COBE data alone, but the small range of angular scales covered by COBE limits the precision of these methods. 
Edward L. Wright