With the increasing importance of producing large-scale labeled datasets for training, testing and validation, services such as Amazon Mechanical Turk (MTurk) are becoming more and more popular to replace the tedious task of manual labeling finished by hand. However, annotators in these crowdsourcing services are known to exhibit different levels of skills, consistencies and even biases, making it difficult to estimate the ground truth class label from the imperfect labels provided by these annotators. To solve this problem, we present a discriminative approach to infer the ground truth class labels by mapping both annotators and the tasks into a low-dimensional space. Our proposed model is inherently combinatorial and therefore does not require any prior knowledge about the annotators or the examples, thereby providing more simplicity and computational efficiency than the state-of-the-art Bayesian methods. We also show that our lightweight approach is, experimentally on real datasets, more accurate than either majority voting or weighted majority voting.