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Data Quality from Crowdsourcing: A Study of Annotation Selection Criteria

Annotation acquisition is an essential step in training supervised classifiers. However, manual annotation is often time-consuming and expensive. The possibility of recruiting annotators through Internet services (e.g., Amazon Mechanic Turk) is an appealing option that allows multiple labeling tasks to be outsourced in bulk, typically with low overall costs and fast completion rates. In this paper, we consider the difficult problem of classifying sentiment in political blog snippets. Annotation data from both expert annotators in a research lab and non-expert annotators recruited from the Internet are examined. Three selection criteria are identified to select high-quality annotations: noise level, sentiment ambiguity, and lexical uncertainty. Analysis confirm the utility of these criteria on improving data quality. We conduct an empirical study to examine the effect of noisy annotations on the performance of sentiment classification models, and evaluate the utility of annotation selection on classification accuracy and efficiency.


Introduction
Crowdsourcing (Howe, 2008) is an attractive solution to the problem of cheaply and quickly acquiring annotations for the purposes of constructing all kinds of predictive models. To sense the potential of crowdsourcing, consider an observation in von Ahn et al. (2004): a crowd of 5,000 people playing an appropriately designed computer game 24 hours a day, could be made to label all images on Google (425,000,000 images in 2005) in a matter of just 31 days. Several recent papers have studied the use of annotations obtained from Amazon Mechanical Turk, a marketplace for recruiting online workers (Su et al., 2007;Kaisser et al., 2008;Kittur et al., 2008;Sheng et al., 2008;Snow et al., 2008;Sorokin and Forsyth, 2008).
With efficiency and cost-effectiveness, online recruitment of anonymous annotators brings a new set of issues to the table. These workers are not usually specifically trained for annotation, and might not be highly invested in producing good-quality annotations. Consequently, the obtained annotations may be noisy by nature, and might require additional validation or scrutiny. Several interesting questions immediately arise in how to optimally utilize annotations in this setting: How does one handle differences among workers in terms of the quality of annotations they provide? How useful are noisy annotations for the end task of creating a model? Is it possible to identify genuinely ambiguous examples via annotator disagreements? How should these considerations be treated with respect to intrinsic informativeness of examples? These questions also hint at a strong connection to active learning, with annotation quality as a new dimension to the problem.
As a challenging empirical testbed for these issues, we consider the problem of sentiment classification on political blogs. Given a snippet drawn from a political blog post, the desired output is a polarity score that indicates whether the sentiment expressed is positive or negative. Such an analysis provides a view of the opinion around a subject of interest, e.g., US Presidential candidates, aggregated across the blogsphere. Recently, sentiment analy-sis is emerging as a critical methodology for social media analytics. Previous research has focused on classifying subjective-versus-objective expressions (Wiebe et al., 2004), and also on accurate sentiment polarity assignment (Turney, 2002;Yi et al., 2003;Pang and Lee, 2004;Sindhwani and Melville, 2008;Melville et al., 2009).
The success of most prior work relies on the quality of their knowledge bases; either lexicons defining the sentiment polarity of words around a topic (Yi et al., 2003), or quality annotation data for statistical training. While manual intervention for compiling lexicons has been significantly lessened by bootstrapping techniques (Yu and Hatzivassiloglou, 2003;Wiebe and Riloff, 2005), manual intervention in the annotation process is harder to avoid. Moreover, the task of annotating blog-post snippets is challenging, particularly in a charged political atmosphere with complex discourse spanning many issues, use of cynicism and sarcasm, and highly domain-specific and contextual cues. The downside is that high-performance models are generally difficult to construct, but the upside is that annotation and data-quality issues are more clearly exposed.
In this paper we aim to provide an empirical basis for the use of data selection criteria in the context of sentiment analysis in political blogs. Specifically, we highlight the need for a set of criteria that can be applied to screen untrustworthy annotators and select informative yet unambiguous examples for the end goal of predictive modeling. In Section 2, we first examine annotation data obtained by both the expert and non-expert annotators to quantify the impact of including non-experts. Then, in Section 3, we quantify criteria that can be used to select annotators and examples for selective sampling. Next, in Section 4, we address the questions of whether the noisy annotations are still useful for this task and study the effect of the different selection criteria on the performance of this task. Finally, in Section 5 we present conclusion and future work.

Annotating Blog Sentiment
This section introduces the Political Blog Snippet (PBS) corpus, describes our annotation procedure and the sources of noise, and gives an overview of the experiments on political snippet sentiments.

The Political Blog Snippet Corpus
Our dataset comprises of a collection of snippets extracted from over 500,000 blog posts, spanning the activity of 16,741 political bloggers in the time period of Aug 15, 2008 to the election day Nov 4, 2008. A snippet was defined as a window of text containing four consecutive sentences such that the head sentence contained either the term "Obama" or the term "McCain", but both candidates were not mentioned in the same window. The global discourse structure of a typical political blog post can be highly complicated with latent topics ranging from policies (e.g., financial situation, economics, the Iraq war) to personalities to voting preferences. We therefore expected sentiment to be highly nonuniform over a blog post. This snippetization procedure attempts to localize the text around a presidential candidate with the objective of better estimating aggregate sentiment around them. In all, we extracted 631,224 snippets. For learning classifiers, we passed the snippets through a stopword filter, pruned all words that occur in less than 3 snippets and created normalized term-frequency feature vectors over a vocabulary of 3,812 words.

Annotation Procedure
The annotation process consists of two steps: Sentiment-class annotation: In the first step, as we are only interested in detecting sentiments related to the named candidate, the annotators were first asked to mark up the snippets irrelevant to the named candidate's election campaign. Then, the annotators were instructed to tag each relevant snippet with one of the following four sentiment polarity labels: Positive, Negative, Both, or Neutral.
Alignment annotation: In the second step, the annotators were instructed to mark up whether each snippet was written to support or oppose the target candidate therein named. The motivation of adding this tag comes from our interest in building a classification system to detect positive and negative mentions of each candidate. For the snippets that do not contain a clear political alignment, the annotators had the freedom to mark it as neutral or simply not alignment-revealing.
In our pilot study many bloggers were observed to endorse a named candidate by using negative ex-pressions to denounce his opponent. Therefore, in our annotation procedure, the distinction is made between the coding of manifest content, i.e., sentiments "on the surface", and latent political alignment under these surface elements.

Agreement Study
In this section, we compare the annotations obtained from the on-site expert annotators and those from the non-expert AMT annotators.

Expert (On-site) Annotation
To assess the reliability of the sentiment annotation procedure, we conducted an agreement study with three expert annotators in our site, using 36 snippets randomly chosen from the PBS Corpus. Overall agreement among the three annotators on the relevance of snippets is 77.8%. Overall agreement on the four-class sentiment codings is 70.4%.
Analysis indicate that the annotators agreed better on some codings than the others. For the task of determining whether a snippet is subjective or not 1 , the annotators agreed 86.1% of the time. For the task of determining whether a snippet is positive or negative, they agreed 94.9% of the time.
To examine which pair of codings is the most difficult to distinguish, Table 1 summarizes the confusion matrix for the three pairs of annotator's judgements on sentiment codings. Each column describes the marginal probability of a coding and the probability distribution for this coding being recognized as another coding (including itself). As many bloggers use cynical expressions in their writings, the most confusing cases occur when the annotators have to determine whether a snippet is "negative" or "neutral". The effect of cynical expressions on  sentiment analysis in the political domain is also revealed in the second step of alignment annotation. Only 42.5% of the snippets have been coded with alignment coding in the same direction as its sentiment coding -i.e., if a snippet is intended to support (oppose) a target candidate, it will contain positive (negative) sentiment. The alignment coding task has been shown to be reliable, with the annotators agreeing 76.8% of the time overall on the three-level codings: Support/Against/Neutral.

Amazon Mechanical Turk Annotation
To compare the annotation reliability between expert and non-expert annotators, we further conducted an agreement study with the annotators recruited from Amazon Mechanical Turk (AMT). We have collected 1,000 snippets overnight, with the cost of 4 cents per annotation.
In the agreement study, a subset of 100 snippets is used, and each snippet is annotated by five AMT annotators. These annotations were completed by 25 annotators whom were selected based on the approval rate of their previous AMT tasks (over 95% of times). 2 The AMT annotators spent on average 40 seconds per snippet, shorter than the average of two minutes reported by the on-site annotators. The lower overall agreement on all four-class sentiment codings, 35.3%, conforms to the expectation that the non-expert annotators are less reliable. The Turk annotators also agreed less on the three-level alignment codings, achieving only 47.2% of agreement.
However, a finer-grained analysis reveals that they still agree well on some codings: The overall agreement on whether a snippet is relevant, whether a snippet is subjective or not, and whether a snippet is positive or negative remain within a reasonable range: 81.0%, 81.8% and 61.9% respectively.

Gold Standard
We defined the gold standard (GS) label of a snippet in terms of the coding that receives the majority votes. 3 Column 1 in  Table 2: Average prediction accuracy on gold standard (GS) using one-coder strategy and inter-coder agreement.
tion) shows the ratio of the onsite expert annotations that are consistent with the gold standard, and Column 3 (AMT-GS prediction) shows the same for the AMT annotations. The level of consistency, i.e., the percentage agreement with the gold standard labels, can be viewed as a proxy of the quality of the annotations. Among the AMT annotations, Columns 2 (onsite agreement) and 4 (AMT agreement) show the pair-wise intercoder agreement in the on-site expert and AMT annotations respectively. The results suggest that it is possible to take one single expert annotator's coding as the gold standard in a number of annotation tasks using binary classification. For example, there is a 97.4% chance that one expert's coding on the polarity of a snippet, i.e., whether it is positive or negative, will be consistent with the gold standard coding. However, this one-annotator strategy is less reliable with the introduction of non-expert annotators. Take the task of polarity annotation as an example, the intercoder agreement among the AMT workers goes down to 61.9% and the "one-coder" strategy can only yield 71.4% accuracy. To determine reliable gold standard codings, multiple annotators are still necessary when non-expert annotators are recruited.

Annotation Quality Measures
Given the noisy AMT annotations, in this section we discuss some summary statistics that are needed to control the quality of annotations.

Annotator-level noise
To study the question of whether there exists a group of annotators who tend to yield more noisy annotations, we evaluate the accumulated noise level introduced by each of the annotators. We define the noise level as the deviation from the gold standard labels. Similar to the measure of individual error rates pro-posed in (Dawid and Skene, 1979), the noise level of a particular annotator j, i.e., noise(anno j ), is then estimated by summing up the deviation of the annotations received from this annotator, with a small sampling correction for chance disagreement. Analysis results demonstrate that there does exist a subset of annotators who yield more noisy annotations than the others. 20% of the annotators (who exceed the noise level 60%) result in annotations that have 70% disagreement with the gold standard.
In addition, we also evaluate how inclusion of noisy annotators reduces the mean agreement with Gold Standard. The plot (left) in Figure 1 plots the mean agreement rate with GS over the subset of annotators that pass a noise threshold. These results show that the data quality decreases with the inclusion of more untrustworthy annotators.

Snippet-level sentiment ambiguity
We have observed that not all snippets are equally easy to annotate, with some containing more ambiguous expressions. To incorporate this concern in the selection process, a key question to be answered is whether there exist snippets whose sentiment is substantially less distinguishable than the others.
We address this question by quantifying ambiguity measures with the two key properties shown as important in evaluating the controversiality of annotation snippets (Carenini and Cheung, 2008): (1) the strength of the annotators' judgements and (2) the polarity of the annotations. The measurement needs to satisfy the constraints demonstrated in the following snippets: (1) An example that has received three positive codings are more ambiguous than that has received five, and (2) an example that has received five positive codings is more ambiguous than the one that has received four positive and one negative coding. In addition, as some snippets were shown to be difficult to tell whether they contain negative or neutral sentiment, the measure of example ambiguity has to go beyond controversiality and incorporate codings of "neutral" and "both".
To satisfy these constraints, we first enumerated through the codings of each snippet and counted the number of neutral, positive, both, and negative codings: We added (1) one to the positive (negative) category for each positive (negative) coding, (2) 0.5 to the neutral category with each neutral coding, and (3) 0.5 to both the positive and negative categories with each both coding. The strength of codings in the three categories, i.e., str + (snip i ), str neu (snip i ), and str − (snip i ), were then summed up into str(snip i ). The distribution were parameterized with We then quantify the level of ambiguity in the annotator's judgement as follows: where str max is the maximum value of str among all the snippets in the collection. The plot (middle) in Figure 1 shows that with the inclusion of snippets that are more ambiguous in sentiment disambiguation, the mean agreement with Gold Standard decreases as expected.

Combining measures on multiple annotations
Having established the impact of noise and sentiment ambiguity on annotation quality, we then set out to explore how to integrate them for selection. First, the ambiguity scores for each of the snippets are reweighed with respect to the noise level.
where θ(ij) is an indicator function of whether a coding of snip i from annotator j agrees with its gold standard coding. w(exp i ) is thus computed as the aggregated noise level of all the annotators who labeled the ith snippet.
To understand the baseline performance of the selection procedure, we evaluate the the true predictions versus the false alarms resulting from using each of the quality measures separately to select annotations for label predictions. In this context, a true prediction occurs when an annotation suggested by our measure as high-quality indeed matches the GS label, and a false alarm occurs when a high quality annotation suggested by our measure does not match the GS label. The ROC (receiver operating characteristics) curves in Figure 2 reflect all the potential operating points with the different measures.
We used data from 2,895 AMT annotations on 579 snippets, including 63 snippets used in the agreement study. This dataset is obtained by filtering out the snippets with their GS labels as 1 ("irrelevant") and the snippets that do not receive any coding that has more than two votes. Initially, three quality measures are tested: 1noise, 1-ambiguity, 1-confusion. Examination of the snippet-level sentiment codings reveals that some snippets (12%) result in "divisive" codings, i.e., equal number of votes on two codings.
The ROC curves in Figure 2 (a) plot the baseline performance of the different quality measures. Results show that before removing the subset of divisive snippets, the only effective selection criteria is obtained by monitoring the noise level of annotators. Figure 2 (b) plots the performance after removing the divisive snippets. In addition, our ambiguity scores are computed under two settings: (1) with only the polar codings (pos/neg), and (2) with all the four codings (all4codings). The ROC curves reveal that analyzing only the polar codings is not sufficient for annotation selection.
The results also demonstrate that confusion, an integrated measure, does perform best. Confusion is just one way of combining these measures. One may chose alternative combinations -the results here primarily illustrate the benefit of considering these different dimensions in tandem. Moreover, the difference between plot (a) and (b) suggests that removing divisive snippets is essential for the quality measures to work well. How to automatically identify the divisive snippets is therefore important to the success of the annotation selection process.

Effect of lexical uncertainty on divisive snippet detection
In search of measures that can help identify the divisive snippets automatically, we consider the inherent lexical uncertainty of an example. Uncertainty Sampling (Lewis and Catlett, 1994) is one common heuristic for the selection of informative instances, which select instances that the current classifier is most uncertain about. Following on these lines we measure the uncertainty on instances, with the assumption that the most uncertain snippets are likely to be divisive. In particular, we applied a lexical sentiment classifier (c.f. Section 4.1.1) to estimate the likelihood of an unseen snippet being of positive or negative sentiment, i.e., P + (exp i ), P − (exp i ), by counting the sentiment-indicative word occurrences in the snippet. As in our dataset the negative snippets far exceed the positive ones, we also take the prior probability into account to avoid class bias. We then measure lexical uncertainty as follows.
Deviation(snip i ) = 1 C × |(log(P (+))−log(P (−))) +(log(P + (snip i ))−log(P − (snip i )))|, where class priors, P (+) and P (−), are estimated with the dataset used in the agreement studies, and C is the normalization constant. We then examine not only the utility of lexical uncertainty in identifying high-quality annotations, but  also the utility of such measure in identifying divisive snippets. Figure 1 (right) shows the effect of lexical uncertainty on filtering out low-quality annotations. Figure 3 demonstrates the effect of lexical uncertainty on divisive snippet detection, suggesting the potential use of lexical uncertainty measures in the selection process.

Empirical Evaluation
The analysis in Sec. 3 raises two important questions: (1) how useful are noisy annotations for sentiment analysis, and (2) what is the effect of online annotation selection on improving sentiment polarity classification?

Polarity Classifier with Noisy Annotations
To answer the first question raised above, we train classifiers based on the noisy AMT annotations to classify positive and negative snippets. Four different types of classifiers are used: SVMs, Naive Bayes (NB), a lexical classifier (LC), and the lexical knowledge-enhanced Pooling Multinomial classifier, described below.

Lexical Classifier
In the absence of any labeled data in a domain, one can build sentiment-classification models that rely solely on background knowledge, such as a lexicon defining the polarity of words. Given a lexicon of positive and negative terms, one straightforward approach to using this information is to measure the frequency of occurrence of these terms in each document. The probability that a test document belongs to the positive class can then be computed as P (+|D) = a a+b ; where a and b are the number of occurrences of positive and negative terms in the document respectively. A document is then classified as positive if P (+|D) > P (−|D); otherwise, the document is classified as negative. For this study, we used a lexicon of 1,267 positive and 1,701 negative terms, as labeled by human experts.

Pooling Multinomials
The Pooling Multinomials classifier was introduced by the authors as an approach to incorporate prior lexical knowledge into supervised learning for better text classification. In the context of sentiment analysis, such lexical knowledge is available in terms of the prior sentiment-polarity of words. Pooling Multinomials classifies unlabeled examples just as in multinomial Naïve Bayes classification (McCallum and Nigam, 1998), by predicting the class with the maximum likelihood, given by argmax c j P (c j ) i P (w i |c j ); where P (c j ) is the prior probability of class c j , and P (w i |c j ) is the probability of word w i appearing in a snippet of class c j . In the absence of background knowledge about the class distribution, we estimate the class priors P (c j ) solely from the training data. However, unlike regular Naïve Bayes, the conditional probabilities P (w i |c j ) are computed using both the labeled examples and the lexicon of labeled features. Given two models built using labeled examples and labeled features, the multinomial parameters of such models can be aggregated through a convex combination, P (w i |c j ) = αP e (w i |c j )+(1−α)P f (w i |c j ); where P e (w i |c j ) and P f (w i |c j ) represent the probability assigned by using the example labels and feature labels respectively, and α is the weight for combining these distributions. The weight indicates a level of confidence in each source of information, and can be computed based on the training set accuracy of the two components. The derivation and details of these models are not directly relevant to this paper, but can be found in (Melville et al., 2009

Results on Polarity Classification
We generated a data set of 504 snippets that had 3 or more labels for either the positive or negative class. We compare the different classification approaches using 10-fold cross-validation and present our results in Table 3. Results show that the Pooling Multinomial classifier, which makes predictions based on both the prior lexical knowledge and the training data, can learn the most from the labeled data to classify sentiments of the political blog snippets. We observe that despite the significant level of noise and ambiguity in the training data, using majority-labeled data for training still results in classifiers with reasonable accuracy.

Effect of Annotation Selection
We then evaluate the utility of the quality measures in a randomly split dataset (with 7.5% of the data in the test set). We applied each of the measures to rank the annotation examples and then divide them into 4 equal-sized training sets based on their rankings. For example, Noise-Q1 contains only the least noisy quarter of annotations and Q4 the most noisy ones.
Results in Table 4 demonstrate that the classification performance declines with the decrease of each quality measure in general, despite exceptions in the subset with the highest sentiment ambiguity (Ambiguity-Q4), the most noisy subset Q4 (Noise-Q4), and the subset yielding less overall confusion (Confusion-Q2). The results also reveal the benefits of annotation selection on efficiency: using the subset of annotations predicted in the top quality quarter achieves similar performance as using the whole training set. These preliminary results suggest that an active learning scheme which considers all three quality measures may indeed be effective in improving label quality and subsequent classification accuracy.

Conclusion
In this paper, we have analyzed the difference between expert and non-expert annotators in terms of annotation quality, and showed that having a single non-expert annotator is detrimental for annotating sentiment in political snippets. However, we confirmed that using multiple noisy annotations from different non-experts can still be very useful for modeling. This finding is consistent with the simulated results reported in (Sheng et al., 2008). Given the availability of many non-expert annotators ondemand, we studied three important dimensions to consider when acquiring annotations: (1) the noise level of an annotator compared to others, (2) the inherent ambiguity of an example's class label, and (3) the informativeness of an example to the current classification model. While the first measure has been studied with annotations obtained from experts (Dawid and Skene, 1979;Clemen and Reilly, 1999), the applicability of their findings on non-expert annotation selection has not been examined.
We showed how quality of labels can be improved by eliminating noisy annotators and ambiguous examples. Furthermore, we demonstrated the quality measures are useful for selecting annotations that lead to more accurate classification models. Our results suggest that a good active learning or online learning scheme in this setting should really consider all three dimensions. The way we use to integrate the different dimensions now is still preliminary. Also, our empirical findings suggest that some of the dimensions may have to be considered separately. For example, due to the divisive tendency of the most informative examples, these examples may have to be disregarded in the initial stage of annotation selection. Also, the way we use to combine these measures is still preliminary. The design and testing of such schemes are avenues for future work.