It may also be desirable to ensure that the source and target training instances are similar in terms of their observed features; this is the idea behind the instance weighting approach to domain adaptation (Jiang and Zhai, 2007) . Motivated by this idea, we require that sampled instances from the source domain have a cosine similarity of at least τ with at least one target domain instance<cite> (Rutherford and Xue, 2015)</cite> .