Recent years have seen a growing interest in research into patent retrieval. One of the key issues in conducting information retrieval (IR) research is meaningful evaluation of the effectiveness of the retrieval techniques applied to task under investigation. Unlike many existing well explored IR tasks where the focus is on achieving high retrieval precision, patent retrieval is to a significant degree a recall focused task. The standard evaluation metric used for patent retrieval evaluation tasks is currently mean average precision (MAP). However this does not reflect system recall well. Meanwhile, the alternative of using the standard recall measure does not reflect user search effort, which is a significant factor in practical patent search environments. In recent work we introduce a novel evaluation metric for patent retrieval evaluation (PRES) [ 13]. This is designed to reflect both system recall and user effort. Analysis of PRES demonstrated its greater effectiveness in evaluating recall-oriented applications than standard MAP and Recall. One dimension of the evaluation of patent retrieval which has not previously been studied is the effect on reliability of the evaluation metrics when relevance judgements are incomplete. We provide a study comparing the behaviour of PRES against the standard MAP and Recall metrics for varying incomplete judgements in patent retrieval. Experiments carried out using runs from the CLEF-IP 2009 datasets show that PRES and Recall are more robust than MAP for incomplete relevance sets for this task with a small preference to PRES as the most robust evaluation metric for patent retrieval with respect to the completeness of the relevance set.