下文选自上海外语教育出版社引进出版的经典英语教学法著作《词汇：描述、习得与教学》，翻译武太白。
How many words are needed to do the things a language user needs to do?
Although a language makes use of a large number of words, not all of these words are equally useful. One measure of usefulness is word frequency, that is, how often the word occurs in normal use of the language. From the point of view of frequency, the word “the” is a very useful word in English. It occurs so frequently that about 7 per cent of the words on a page of written English and the same proportion of the words in a conversation are repetitions of the word “the”. Look back over this paragraph and you will find an occurrence of “the” in almost every line.
The good news for second language learners and second language teachers is that a small number of the words of English occur very frequently and if a learner knows these words, that learner will know a very large proportion of the running words in a written or spoken text. Most of these words are content words and knowing enough of them allows a good degree of comprehension of a text. Here are some figures showing what proportion of a text is covered by certain numbers of high frequency words.
Table 1 Vocabulary size and text coverage in the Brown corpus
Vocabulary size | Text coverage |
1,000 | 72.0% |
2,000 | 79.7% |
3,000 | 84.0% |
4,000 | 86.8% |
5,000 | 88.7% |
6,000 | 89.9% |
15,851 | 97.8% |
(taken from Francis and Kucera, 1982)
The figures in Table 1 refer to written texts and are from Francis and Kucera (1982) which is a very diverse corpus of over 1,000,000 running words made up of 500 texts of around 2,000 running words long. As we shall see, the more diverse the texts in a corpus are, the greater the number of different words, and the high frequency words cover slightly less of the text, so these figures are a conservative estimate. The figures in the last line of the table are from Kucera (1982). The Collins COBUILD English Language Dictionary (1987) claims that 15,000 words cover 95 per cent of the running words of their corpus. The figures in Table 1 are for lemmas and not word families. (A lemma is a base word and its inflected forms.) Word families would give fractionally higher coverage. Table 1 assumes that high frequency words are known before lower frequency words and shows that knowing about 2,000 word families gives near to 80 per cent coverage of written text. The same number of words gives greater coverage of informal spoken text - around 96 per cent (Schonell, Meddleton and Shaw, 1956). (McCarthy and Carter discuss other differences between spoken and written discourse in the next chapter.)
With a vocabulary size of 2,000 words, a learner knows 80 per cent of the words in a text which means that one word in every five (approximately two words in every line) are unknown. Research by Liu Na and Nation (1985) has shown that this ratio ot unknown to known words is not sufficient to allow reasonably successful guessing of the meaning of the unknown words. At least 95 per cent coverage is needed for that. Research by l.aufer (1988a) suggests that 95 per cent coverage is sufficient to allow reasonable comprehension of a text. A larger vocabulary size is clearly better. Table 2 is based on research by Hirsh and Nation (1992) about novels written for teenage or younger readers.
The Hirsh and Nation (1992) study looked at such novels because they might provide the most favourable conditions for second language learners to read unsimplified texts. These conditions could come about because they are aimed at a non-adult audience and thus there may be a tendency for the writer to use simpler vocabulary, and because a continuous novel on one topic by one writer provides opportunity for the repetition of vocabulary. Table 2 shows that under favourable conditions, a vocabulary size of 2,ooo to 3,000 words provides a very good basis for language use.
Table 2 Vocubulary size and coverage in novels for teenagers
Vocabulary size | % coverage | Desity of unknown words |
2,000 words | 90 | 1 in every 10 |
2,000 + proper nouns | 93.7 | 1 in every 16 |
2,600 words | 96 | 1 in every 25 |
5,000 words | 98.5 | 1 in every 67 |
The significance of this information is that although there are well over 54,000 word families in English, and although educated adult native speakers know around 20,000 of these word families, a much smaller number ot words, say between 3-5,000 word families is needed to provide a basis for comprehension. It is possible to make use of a smaller number, around 2-3,000 for productive use in speaking and writing. Hazenburg and Hulstijn (1996), however, suggest a figure nearer to 10,000 for Dutch as a second language.
Sutarsyah, Nation and Kennedy (1994) found that a single long economics text was made up of 5,438 word families and a corpus of similar length made up of diverse short academic texts contained 12,744 word families. Within narrowly focused areas of interest, such as in an economics text, a much smaller vocabulary is needed than if the reader wishes to read a wide range of texts on a variety of different topics.
尽管一门语言用到的单词数是很大的，却并不是所有这些单词都一样有用。对“有用性”的一种量度就是单词出现频率，即一个单词在语言的正常使用中有多么经常出现。从出现频率的角度来看，单词“the”是英语中非常有用的一个词。它出现得如此频繁，以至于英语书面语一页中有7%都是这个词，口语中的重复频率也一样。回头看看本段，你在几乎每一行都找到“the”的身影。
对第二语言学习者和教师来说，好消息是出现频率非常高的英语单词为数不多，只要学习者学会了这些词，他们就能懂得书面或口语文本中的大部分行文。这些单词中的绝大部分都是实义词，这些词只要认识得够多，就能很好地理解一篇文本。下面是一些数字，表明高频词掌握数量与文本覆盖率的关系（图表见上文）：
表1中的数字所指为书面文本，来自弗朗西斯和库切拉（1982），这是一个五花八门的语料库，有超过100万词，由500篇各约2000词的文本组成。我们会看到，语料库的文本内容越是多样化，不同的单词数量就越多，高频词所能覆盖的文本比率相应也就略少，所以这些数字是比较保守的估计。表格最后一行的数字来自库切拉（1982）。科林斯COBUILD英语词典（1987）称15000词覆盖了其语料库行文的95%。表1的数字计算的是词元而非词族。（“词元”是基础词及其语法变化形式的统称。）如按照词族计算，则同样的数字会覆盖略微高一点比例的内容。表1的假定是，高频词要比低频词先认识，这份表格也表明，认识2000个词族就能懂得书面文本的80%。同样多的词族能够覆盖的非正式口语文本比例则高得多——96%左右（肖奈尔，麦德尔顿和肖，1956）。（下一章麦卡锡和卡特探讨的是口语和书面语的其他不同。）
学习者如有2000词的词汇量，就能懂得文本中80%的内容，即每五个词中有一个词是不会的（大约每行两个词）。刘娜（音）和纳辛（1985）已经表明，这种生词率还不足以使学习者在猜测生词意义方面取得不错的成绩。要想做到这一点，至少要有95%的词汇覆盖率。劳弗尔（1988a）所做的研究显示，（学习者有）95%的词汇覆盖率，就足以取得还算不错的文本理解。显然，词汇量大些会更好。表二基于希尔施和纳辛（1992）的研究，内容是为青少年或年龄更小的读者所创作的小说。
希尔施和纳辛（1992）的研究内容是这类小说，因为其能够为二语学习者提供最有利的条件，让他们阅读到没有经过简化的文本。之所以有这样的有利条件，是因为这类小说的目标读者不是成年人，所以作者可能倾向于使用更简单的词汇，并且，同一位作者就单一主题的创作连贯性强，提供了（学习者接触）词汇反复的机会。表2表明，在有利的条件下，2000到3000词就能为语言运用打下良好基础。
这一发现的意义在于，尽管英语中有远超54000个词族，尽管受过教育的本国语者能懂得其中的20000个词族，但只需要掌握小得多的词汇量，比如说3000到5000词族，即可为理解打下基础。在说和写中，可以利用再小一点的词汇量，两三千词左右，即可进行输出性运用。不过，哈增伯格和胡尔斯蒂金（1996）提出，荷兰语的二语学习者要学会将近10000词（才能理解和运用）。
苏斯塔尔西亚，纳辛和肯尼迪（1994）发现，一份较长的经济学文本由5438个词族组成，而由多样化的较短学术文本组成的相近长度的语料库包含12744个词族。在高度聚焦的话题领域，比如在经济学文本中，所需要的词汇量要比阅读多种不同话题形成的多篇文本所需要的词汇量小得多。
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