別記様式第6
論 文 審 査 の 要 旨(Summary of Dissertation Evaluation)
博士の専攻分野の名称

博士（学 術）(Major Field of Ph.D.)

学 位 授 与の要 件 学位規則第４条第○1 ・２項該当

氏名(CandidateName)

Thwin Myint Myint Maw

論 文 題 目 (Title of Dissertation)
Investigating Factors Affecting Lexical Diversity Measure Predictions of Writing and Speaking
Proficiency: Word-Counting Criteria, L1 Background, Language Proficiency, and Text Length
論文審査担当者 (The Dissertation Committee)
主査
准教授
Jon Clenton
審査委員
教授
Simon Fraser
審査委員
准教授
Noriko Yamane
審査委員
准教授
Masahiro Shinya
審査委員
准教授Peter Howell〔論文審査の要旨〕 (Summary of the Dissertation Evaluation)

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This thesis addresses important gaps in Lexical Diversity (LD) research. The thesis investigates
whether LD measures predict inter- and intra-group writing variability under a controlled text
length (200 words) and for a specific L1 background (Chinese). It examines the extent to which
LD measures predict speaking proficiency based on using different constant spoken-sample text
lengths (200 to 450 words). It examines the extent to which LD measures predict writing andspeaking using different-word counting techniques, focusing on the utility of the flemma count (abase word and its inflections under different word classes as the same types).
The first experiment was based on an entire population (N = 194 L2 English writers from mixed
L1 backgrounds). It investigated the extent to which LD measures could discriminate between
the three IELTS-based writing proficiency levels (6.5, 7, 7.5) under a controlled text length basedon the different analysis units.
The second experiment controlled the L1 background and so examined the extent to which LD
measures predict the writing proficiency of an L1 Chinese L2 English learner group (N = 105).
The third experiment controlled both L1 background and language (writing) proficiency. It
explored the extent to which LD measures predict the writing proficiency of L1 Chinese L2
English learners (N = 103) based on different writing proficiency levels (6.5, 7, 7.5).
The fourth experiment analyzed the different participants (55 L2 English speakers from various
L1 backgrounds). It examined whether LD measures were predictive of the IELTS-based
speaking proficiency levels (6.5, 7, 7.5) based on the different analysis units and text lengths. It
followed similar procedures to the first writing experiment to acquire greater comparability of
the findings of the LD measure predictions of two different language modes (L2 writing andspeaking).

Overall, the four experimental studies’ findings indicate that different analysis units influenced LD
measure predictions of L2 language proficiency. Furthermore, LD measures were stronger
predictors or L2 writing than speaking, and LD measures required longer constant text length for
speaking than for writing to achieve accurate predictability. This thesis concludes that LD measurepredictions of L2 language proficiency depends on these four factors, so future LD research should
consider and control them carefully.
Published papers connected to the thesis include one peer-reviewed journal article (in AssessingWriting). The experimental findings reported in the thesis have received significant praise fromlead researchers in the vocabulary field.
As a result of the above examination, the author of this thesis is found to be fully qualified to beawarded the degree of "Doctor of Philosophy”.
備考 要旨は，1,500字以内とする。
(Note: The summary of the Dissertation should not exceed 500 words.)