Automatic evaluation metrics are often used to compare the quality of different systems. However, a small difference between the scores of two systems does not necessary reflect a real difference between their performance. Because such a difference can be significant or only due to chance, it is inadvisable to use a hard ranking to represent the evaluation of multiple systems. In this paper, we propose a cluster-based representation for quality ranking of Machine Translation systems. A comparison of rankings produced by clustering based on automatic MT evaluation metrics with those based on human judgements shows that such interpretation of automatic metric scores provides dependable means of ordering MT systems with respect to their quality. We report experimental results comparing clusterings produced by BLEU, NIST, METEOR, and GTM with those derived from human judgement (of adequacy and fluency) on the IWSLT-2006 evaluation campaign data.