\begin{table}[tttt]
\centering
\caption{Characteristics of our L2R strategies. }
\label{tab:l2rmethods}

%\resizebox{20cm}{!} {
    \begin{tabular}{|l|l|p{3cm}|p{2.5cm}|p{5cm}|}
    \hline
    Approach    & Ensemble & Training instances  &  Directly optimize evaluation metric? & Generated model                         \\\hline

    $RankSVM$     & -        & Pairs of candidate tags              & no           & Hyperplane                             \\\hline
    
    
    $RF$          & bagging  & Sets of candidate tags                & no           & Set of ``randomized'' decision trees \\\hline
    $MART$        & boosting & Sets of candidate tags                & no           & Set of boosted decision trees        \\\hline
    $\lambda$-$MART$ & boosting & Sets of candidate tags            & yes          & Set of boosted decision trees        \\\hline
    
    
    $ListNet$     & -        & Sets of candidate tags               & no           & Neural network                         \\\hline
    $AdaRank$     & boosting & Sets of candidate tags               & yes          & Sets of weighted weak rankers          \\\hline
    
    $RankBoost$   & boosting & Pairs of candidate tags              & no           & Sets of weighted weak rankers          \\\hline
    
    $GP$          & -        & Sets of candidate tags              & yes          & Any function formed by a given set of operators and attributes \\\hline
    \end{tabular}
%}
\end{table}