Baltic J. Modern Computing, Vol. 6 (2018), No. 2, 173-194 https://doi.org/10.22364/bjmc.2018.6.2.09 Acceptability Rating of Ungrammatical Colloquial Latvian: How Native Speakers Judge Different Error Types Jurģis ŠĶILTERS 1 , Līga ZARIŅA 1 , Eglė ŽILINSKAITĖ-ŠINKŪNIENĖ 2 , Velta SKOLMEISTERE 1 1 University of Latvia, Laboratory for Perceptual and Cognitive Systems at the Faculty of Computing, Faculty of Computing, Raina blvd. 19, Rīga, Latvia 2 Vilnius University, Institute for the Languages and Cultures of the Baltic, Faculty of Philology, Universiteto st. 5, Vilnius, Lithuania jurgis.skilters@lu.lv, liga.zarina@lu.lv, egle.zilinskaite@gmail.com, velta.sk@gmail.com Abstract. The aim of the current study is to test what types of sentences according to their grammatical structure are (a) considered to be more plausible from a native speaker’s view and (b) perceived as better understandable even if not entirely grammatical. A quasi-experimental task in a repeated measures design was conducted. 83 native speakers of Latvian have rated a randomized and balanced set of most typical grammatical errors (together with correct sentences and the filler sentences). Importantly our sample contained all regions of Latvia with different dialects (and even a different writing system). According to our results, errors in usage of verbal prefixes, definite / indefinite endings, and word order errors are the closest to the ratings of correct sentences. The most crucial errors recognized by native speakers are the errors of prepositional usage and word coordination. To our knowledge, this is the first acceptability rating experiment for the Baltic languages. Keywords: grammaticality, acceptability rating, colloquial Latvian, types of grammatical errors 1. The problem Latvian is a fusional language that belongs to morphologically rich but small (in terms of speaker population) languages that are neglected in terms of analysis of their grammatical structure and its correspondence to cognitive processing. Even more under- explored are the features of acceptability of certain types of sentences. In this study we are focusing on a subpart of the problem: we are aiming at exploring native language acceptability of colloquial Latvian which is considered ungrammatical according to traditional view of Latvian grammar. We assume the framework of graded grammaticality and explore which features of grammaticality are more important for the understanding of the sentence and which features of ungrammaticality are tolerated by native speakers (i.e., which are the sentence types that native speakers of Latvian accept as good enough although they are