A blocklist is a list of candidate literal translation errors. 


Blocklist Definition  


- (1) Select from the source text the list of words $s = <s_1, s_2, …, s_N>$ belonging to the annotated idiom.  
- (2) For each word $s_i$, obtain its word translations in the target language using a bilingual dictionary and add them to a blocklist $b_i = <t_1, t_2, …, t_M>$ and create a candidate list of blocklists $B_s = <b_1, b_2, …, b_N>$ 
- (3) For each word in the reference (R), search if it occurs in any of the blocklists $b_i$. If so, remove corresponding blocklist $b_i$ from $B_s$. 
- (4) Check if the hypothesis contains any blocklisted words. If so, mark this hypothesis as having a literal translation error. 
- The final score is the percentage of translations that trigger the blocklist. Since LiTER requires source=side annotations, test data with idioms on the source side are collected, and the spans they occur are annotated. 

LiTER Algorithm

Literal Translation Error Rate (LiTER) is a metric for a model’s literal translation errors. A literal translation error happens when a sentence is wrongly translated literally in the target language. Translation errors are identified by checking if a translation output contains any (automatically created compiled) blocklisted words. 


University of California, Berkeley

Targeted evaluation isolates the idiom translation with word alignments or word edit distance, which measures idiom translation accuracy but doesn’t account for literal translation errors. Shao et al. (2018) proposed a method for estimating these errors’ frequency but it requires hand-crafted blocklists with words corresponding to literal translation errors. 


Targeted Evaluation for NMT of idiomatic expression 


Automatic Targeted Evaluation 


Alignment-based Phrase Translation Evaluation (APT-Eval) extends Fadee et al. (2018) and subword-level metrics to measure idiom translation accuracy. It uses word alignments to find words in the hypothesis and reference sentences which align with the annotated idiom source span and compares the two. 

Alignment-based Phrase Translation Evaluation (APT-Eval) 


APT-Eval is a targeted evaluation metric which measures translation accuracy. It is unable to measure literal translation errors in inaccurate translations, but LiTER is able to. 


LITER vs APT-Eval


Prior research overlooks the fact that different idioms have significantly different frequencies. As a result, models overfitted on over-represented idioms are favored. To address this, targeted evaluation results are reported by macro-averaging over idioms. 


Addressing Varying Idiom Frequencies 


Experiments are done on en-fr and en-es data. For each language pair, the data are concatenated from Europarl v7 (Koehn, 2005), part of the WMT news translation task (Bojar et al., 2014), and TED talk transcripts (Cettolo et al., 2017).


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