\subsection{Critical compilation of experimental and theoretical data on 2s-2p transitions of Li-like ions}

All available experimental data on the resonance lines of Li-like ions have been critically compiled. Systematic errors have been corrected in several sets of old measurements, a few outlying measurements have been excluded, and weighted means of all remaining experimental values have been found. Differences of these weighted experimental means from two sets of advanced quantum-mechanical calculations have been fitted with smooth functions of nuclear charge $Z$. From this fitting, accurate recommended data have been derived for all $Z$ between 6 and 92. These recommended data significantly disagree with both theories. While the discrepancies for low $Z$ (\textless 18) and for high $Z$ (\textgreater 70) can be explained by deficiencies in theoretical treatment of inter-electron correlations and nuclear shapes, the discrepancies of a few sigma in the mid-$Z$ range (20--30) are puzzling.