A NEW MEASURE FOR THAI STUDENTS’ VOCABULARY SIZE: THE ENGLISH-THAI VERSION OF VOCABULARY SIZE TEST

The main objective was to explore the reliability and validity of the bilingual English-Thai version of the vocabulary size test adapted based on Nation and Beglar (2007). The findings indicate that (1) the test is reliable; it produced consistent and stable results, (2) the test is valid; it well measures subjects’ vocabulary size, (3) the words in each level are quite organized; the scores generally decline from the beginning level which is the easiest one (1 1000 word level) to the highest level which is the most difficult one (14 1000 word level), and (4) the test result shows that only 14 word levels are not sufficient to access the subjects’ total vocabulary size; the test should contain more word levels. In all, the bilingual English-Thai version of the vocabulary size test is a reliable and valid measure of Thai students’ vocabulary size.


INTRODUCTION
Vocabulary is an essential aspect of effective language use (Knight, 1994;Laufer, 1986;Schmitt, 2000).Many researchers, for instance, Laufer (1998) and Nation and Meara (2002), found high relationships between language skills and vocabulary knowledge.It means that the lack of learners' vocabulary knowledge would negatively affect their abilities in using the language: reading, listening, writing, and speaking skills.Thus, knowing learners' vocabulary size can be beneficial to all concerned parties, such as teachers and learners.
What are the main advantages of knowing learners' vocabulary size?First, vocabulary size helps teachers decide whether learners have adequate vocabulary for efficient language use or not.As mentioned above, vocabulary is an important factor of language, so it is necessary for learners to acquire enough word size; poor lexical knowledge tends to cause learners' poor language skills.According to Hirsh and Nation (1992) and Hu and Nation (2000), learners need to know 98% of running words in the text for sufficient comprehension, which is about 6000-7000 word families for spoken text and 8000-9000 word families for written text (Nation 2006).Second, knowing learners' word knowledge can help teachers develop language lessons.In order to design good lessons, it is very important for teachers to know learners' language proficiency, including vocabulary proficiency.If data on learners' word size is available, it will be easy for teachers to decide what word levels should be added in the lesson to improve their lexical repertoire and what texts, written or spoken, should fit their vocabulary level.Finally, learners' vocabulary size allows teachers to see learners' progress in vocabulary skill.Learners' word knowledge should largely grow over time.
Many tests for estimating learners' vocabulary knowledge have been proposed by researchers in this field, such as Nation (1983Nation ( , 1990)), Meara and Buxton (1987), and Nation and Beglar (2007).Among those well-known tests, the monolingual English version of the vocabulary size test developed by Nation and Beglar (2007) is one of the well-accepted tests.
The purpose of the monolingual English version of the vocabulary size test developed by Nation and Beglar (2007) is to measure learners' receptive vocabulary size which is a learner's ability to know word meanings while reading and listening.The test is a multiple choice format consisting of fourteen frequency word levels; each word level contains 10 items; the total is 140 items.The words in each frequency word level were chosen based on a frequency count of word families occurring in the British National Corpus (BNC).Each of the items begins with a target word followed by a simple sentence containing the word; a sentence shown in the item doesn't provide any clues for guessing the word meaning.For individual items, there are 4 options, there is only one correct choice which has the closest meaning to the target word and the other three are distractors.Here is the example of Item 10 from the 5 th 1000 word level.10.Bacterium: They didn't find a single bacterium.a. small living thing causing disease b. plant with red or orange flowers c. animal that carries water on its back d.thing that has been stolen and sold to a shop As shown above, alternatives of each item of the original test are written in English language.The language used in some choices contains complicated grammar or difficult vocabulary.It means that, in order to answer some items of the test correctly, learners need to have quite a high knowledge of grammar and vocabulary.According to the example of item 10 from 5 th 1000 word level shown above, the language small living thing causing disease used in option a consists of the reduced clause.The full sentence is "small living thing which causes disease".Moreover, the word "disease" is not an easy word for learners either; it may appear to be an unknown word for them.Therefore, the barriers, not understanding the grammar and language used in the options result in test takers not being able to come up with correct answer due to grammar and lexical complexity.
To avoid these problems, the options in the monolingual English version of the vocabulary size test developed by Nation and Beglar (2007) have been translated into the native language of the test takers, Thai language, which is the native language of Thai people.
There is evidence showing that the original test has a high quality for measuring learners' lexical size (Beglar 2010;Gyllstad 2012).There are no such studies conducted to establish the reliability and validity of the adapted version of the test.Therefore, this present study aimed to investigate the reliability and validity of the revised version of vocabulary size test.Nation and Beglar (2007).This test version keeps all aspects of the original monolingual version excluding the language in the choices.The translation was approved by 2 experienced translation specialists.In addition, the extra option "I don't know" was added as the fifth alternative of each item.This added option would prevent learners from guessing; the guessing can distort the real lexical size of them.The example of Item 10 from the 5 th 1000 word level is shown below.Nation and Beglar (2007).The original version of vocabulary test is a multiple-option format.There are 14 word levels in total; each level consists of 10 words; the total amount of items of the test is 140.The organization of individual words in the test is based on word difficulties, a highest frequency of word families revealed in the British National Corpus is arranged in the first word level of the test and the lower frequency of word family is arranged in the last word level.Each item begins with a target word followed by a non-defining phrase or sentence with the goal word.

The bilingual English-Thai version of the vocabulary size test used in this present research is an adapted version of the original version designed by
The revised version of the receptive vocabulary size test keeps all features from the original test except for the language used in each alternative; they were translated from English to Thai.The language translated was checked for correctness and appropriateness by 2 translation specialists.In addition, an extra fifth choice was added in the test in order to prevent learners from guessing.Here is the comparison between the monolingual English version test and the bilingual English-Thai version test of Item 10 from the 5 th 1000 word level.

The original monolingual version
The adapted bilingual version 10. Bacterium: They didn't find a single 10.Bacterium: They didn't find a single bacterium.bacterium a. small living thing causing disease The researcher informed 331 participants of this study about the purpose of the study and test instruction.Then, the bilingual English-Thai version of the vocabulary size test was administered to all participants.Subjects had no time limit for completing the test, because we want to assess their vocabulary size, not test their speed.

MEASURING THE VOCABULARY SIZE
In terms of the scoring of the vocabulary size test, a correct answer gets 1 point; an incorrect receives zero.To explore the subjects' vocabulary size, the total score from all 14 word levels is multiplied by 100.For instance, a learner who gets score of 55 out of 140 will have a vocabulary size of 5,500 word families.

STATISTICAL ANALYSIS
Cronbach's alpha was applied to indicate the consistency and stability of the test.Pearson's correlation coefficient was used to analyze the value of the relationship between language proficiency and vocabulary knowledge.Moreover, descriptive statistics were used to find the scores of each word level.

Research question 1: Does the bilingual English-Thai version of the vocabulary size test perform consistently and stably?
To analyze the reliability, consistency and stability of the bilingual English-Thai version of the vocabulary size test, Cronbach's alpha was used.The data used to analyze the reliability was collected from 331 subjects.The interpretation of the reliability level of the test is based on Hatcher (1994).The Chonbach's alpha of above .70 is the acceptable value (Hatcher 1994).
According to Table 1, the reliability of the vocabulary size test, a total of 14 1000 word levels, is found to be high above the acceptable level (a = .903).When the data of each word level was interpreted, the value of individual word levels is also reviewed at the above the acceptable value.The Chonbach's alpha of each word level ranged from .881 to .892; the 5 th 1000 word level has the lowest value and the 1 st 1000 word level has the highest value.
It indicates that the bilingual English-Thai version of the vocabulary size test is reliable to measure the learners' vocabulary size.It produces consistent and stable results.It has been admitted by many researchers (e.g.Laufer 1986;Hermann 2003) that vocabulary is a crucial element in language.To be able to read, listen, write, and speak effectively, word knowledge would be a key factor.Thus, the level of learners' English proficiency would correlate with their level of vocabulary knowledge.In other words, high English proficiency learners receive high scores from the vocabulary size test and low proficiency learners have low scores from the test.In order to see how well the test measures learners' vocabulary size, the correlation between learners' vocabulary size and proficiency is used.
It should be noted that the bilingual English-Thai version of the vocabulary size test under investigation is a test of receptive vocabulary size.Receptive vocabulary is related to receptive skills, reading and listening (Nation 1990;Schmitt 2010).In order to be sure that the bilingual English-Thai version of the vocabulary size test is a valid test, this present study aims to explore the relationship between the subjects' receptive vocabulary size and their receptive skills, their abilities to recognize word meanings while reading and listening.Therefore, the data of 331 subjects' English proficiency in both reading and listening skill, was collected 2 weeks before the administration of the bilingual English-Thai version of the vocabulary size test.The subjects' reading and listening skills are taken from the PSU English Tests, officially used and administered annually to all third-year students at Prince of Songkla University.
As shown in Table 2, the correlation between the subjects' English receptive knowledge and their lexical size is significant at a high level (r = .806,p<.01).Moreover, the subjects' reading and listening scores are also significantly correlated with their vocabulary knowledge at a high level (r = .785and .687),respectively.
Based on these results, the high relationship between the subjects' reading and listening proficiency and their vocabulary size indicates that subjects with higher English proficiency would have higher vocabulary size, and subjects with lower English knowledge would have lower word knowledge.
As mentioned earlier, vocabulary knowledge plays an important factor in reflecting learners' receptive proficiency.As the correlation analysis pointed out above, it has been proven that this vocabulary size test can reliably assess subjects' word knowledge.
In conclusion, this bilingual English-Thai version of vocabulary size test performs well in measuring subjects' receptive vocabulary size.Research question 3: Do the difficulties of words in the test decline from the high frequency word level to the lower one?
In terms of the arrangement of words in the test, the difficulties of words must be related to the word levels of the test; the 1 st 1000 word level, which is the first word level among 14 levels, should contain the words with the lowest difficulty in all levels, and the 14 th 1000 word level, which is the last word level of all, consists of the words with the highest difficulty.That is, learners tend to know the high frequency words more than the low frequency words; they are likely to comprehend the words in 1 st 1000 level better than the ones in 2 nd 1000 level; they know more words in 2 nd 1000 level than the ones in 3 rd 1000 level, and so on.Thus, if the words in each level are ordered appropriately regarding their difficulties, Thai subjects' scores should drop from the 1 st 1000 word level to 14 th word level.
Table 3 and Figure 1 contain the data of the subjects' vocabulary size for each of the 14 word levels.Although the results reveal that the decline of the subjects' word scores is not perfectly consistent, the tendency of their scores generally falls from the 1 st 1000 level to the 14 th 1000 level.The highest vocabulary score of the subjects is on the 1 st word level and the lowest score is on the 13 th word level.The mean scores from the 1 st 1000 word level to 5 th 1000 level properly decrease -subjects acquire the word sizes of 846, 647, 612, 558, 436 for the 1 st , 2 nd , 3 rd , 4 th , and 5 th word level, respectively.The subjects' score for the 8 th word level is in the sixth place (402), followed by the scores in 6 th 1000 word level (353), 7 th (357), 11 th (350), 12 th (307), 14 th (250), 9 th (246), 10 th (216), 13 th levels (217), respectively.The inconsistency appearing on the latter levels of the test may have occurred because of a number of reasons.First, learners might have guessed the meanings of unknown words, especially the words belonging to the high word levels.Even though an extra choice of "I don't know" is offered for each item, learners didn't tend to choose this option; they preferred to choose other alternatives instead.As a result, they might gain in score from their guessing.
Second, some words in the high level are very common in Thai context; some words have already become Thai words.Here is an example of common words in 7 th -14 th levels.
The high scores appearing abnormally on the 8 th , 11 th , and 12 th levels may have been caused by those common words.
In total, the difficulties of words in the test tend to decrease from the high word level to the low word level.It means that the words in the test are quite organized.The low difficulty words are arranged in the low word levels and the high difficulty ones are arranged in the high word levels.
Research question 4: Are the 14 word levels in the vocabulary size test enough to assess learners' total word size?
As Table 4 shows, most subjects gain some scores in each word level of the test.There is only a small amount of them that receives zero score.
When the scores of the subjects in each word level are considered, the findings show that no learners have the score of zero in 1 st -4 th word levels.There are only 1.51 percent of learners having the score of zero in 5 th word level; 2.11 percent have zero score in 6 th word level; 1.21 percent have zero score in 7 th word level; 3.02 percent have zero score in 8 th word level; 12.99 percent have zero score in 9 th word level; 16.62 percent have zero score in 10 h word level; 0.91 percent have zero score in 11 th word level; 6.95 percent have zero score in 12 th word level; 10.57 percent have zero score in 13 th word level; and 7.85 percent have zero score in 14 th word level.
According to the results, the subjects tend to receive some scores in the further word levels.It seems that the 14 word levels are not adequate to estimate subjects' total word size.Thus, to measure the subjects' real vocabulary size, it is necessary to have more word levels in the test.

CONCLUSION
The research findings are summarized as follows: 1.The bilingual English-Thai version of vocabulary size test seems to be a reliable test to measure Thai learners' receptive vocabulary size.It has a consistency and repeatability of measuring learners' vocabulary size.
2. The bilingual English-Thai version of vocabulary size test is a valid test; the test well measures the subjects' vocabulary size.The findings reveal that there is a high relationship between subjects' English receptive skills and their vocabulary knowledge.In other words, the vocabulary size test can diagnose learners of different English proficiency; learners with high English proficiency receive a high English lexical size from the test and vice versa.
3. The words in the test are moderately organized; the words with the low difficulties appear in the low word levels and vice versa.The findings show that the subjects' vocabulary scores generally decrease from the 1 st 1000 word level to the 14 th 1000 word level.There were, however, some few words in the high word levels producing inconsistency due to learners' familiarity with the words in their Thai context.
4. It is necessary to have further word levels in the vocabulary size test.
According to the findings, the subjects tend to have some scores on every word level: 1 st -14 th word levels, so they have a chance to receive more vocabulary scores on the higher word levels.Adding more word levels may positively affect learners' vocabulary size, some learners may gain more scores on the higher levels.
As mentioned above, it can be concluded that the bilingual English-Thai version of the vocabulary size test is reliable and valid to measure Thai learners' receptive vocabulary size.Moreover, the items in the test are found to be generally well organized.More word levels should be added in the vocabulary size test.

FURTHER STUDIES
The results showed that 14 word levels are insufficient for measuring learners' vocabulary size.For further investigation, research should be conducted to explore how many word levels should be added in the vocabulary size test.

Figure 1 .
Figure 1.Vocabulary size in each word level.

Table 1 .
Reliability of the vocabulary size test.
Research question 2: Does the test well measure learner's vocabulary size?

Table 2 .
Relationship between vocabulary size and English proficiency.
** Significant at the .01level

Table 3 .
Descriptive Statistics of the test.

Table 4 .
Scores in each word level.