Mostly, the distinction between receptive and productive knowledge is considered as the distinction between receptive skills (listening and reading) and productive skills (speaking and writing) (e. g. Crow, 1986; Laufer & Goldstein, 2004). According to Nation (2001), the terms “receptive” and “productive" are not completely suitable because there are productive features in the receptive skills since when we listen or read we produce meaning. As Milton (2009, p.13) has pointed out, `good passive skills often require the reader or the listener to actively anticipate the words that will occur'.

Not all researchers define the receptive/productive dichotomy in the same way, and it has created problems making comparisons between these two kinds of knowledge (Read, 2000). Corson (1995) uses the terms “active" and “passive” to refer to productive and receptive vocabularies, and his description is strongly based on the idea of use and not solely on degrees of knowledge. Also, Laufer, Elder, Hill, and Congdon (2004) describe Receptive knowledge as retrieval of the word's form, and productive knowledge as retrieval of the word's meaning. Likewise, Webb (2008) defines Receptive vocabulary knowledge as the ability to recognize the form of a word and to define or find a synonym for it, while Productive vocabulary is the ability to recall the form and meaning of a foreign language word. These definitions restrict the concept of Receptive and Productive vocabulary knowledge to the form and meaning aspects.

However, Henriksen (1999) categorizes lexical knowledge in three components:

1) a partial-to-precise knowledge dimension where levels of knowledge are operationalized as degrees of understanding;

2) a depth - of-knowledge dimension which reveals the multi-aspect nature of word knowledge, and extends to a word's syn - tagmatic and paradigmatic relations with other words; and 3) a receptive-productive dimension which refers to the mastery levels of vocabulary knowledge reflected in the learners' comprehension and productive ability.

Additionally, Nation (2001) distinguished the three following types of knowledge: knowledge of form, meaning and use, and then subdivided each of these three categories into three subcategories. For instance, the knowledge of form is further divided into spoken, written and word parts. Nation suggests that receptive and productive knowledge of a word should cover all aspects of what is involved in knowing a word. In other words, each of the aspects in the partial-precise and depth dimensions can be mastered at a receptive or productive level for use (Zhong, 2012). Therefore, Zhong (2012, p.29) adapting Nation's definition of receptive and productive vocabulary knowledge, argues that `receptive vocabulary knowledge can be conceptualized as the comprehension ability in reading and listening, and productive vocabulary knowledge can be conceptualized as the ability to apply the word appropriately to fit into a context in writing and speaking'.

Receptive and productive knowledge: a continuum or dichotomous units

As Laufer and Goldstein (2004) have pointed out, another debatable issue about the receptive and productive vocabulary knowledge is that whether the distinction between these two is dichotomous or it constitutes a continuum. Some researchers such as Melka (1997) suggest that they are placed on a continuum. To put it in other words, as learners learn about lexical items, Receptive knowledge gradually moves toward Productive knowledge. Although this viewpoint is confirmed by many researchers, the threshold at which receptive knowledge turns into productive is not determined (Laufer & Goldstein, 2004; Read, 2000; Schmitt, 2010). On the other hand, Meara (1997) suggests that the two types of knowledge represent different association types between lexical items rather than being on a continuum. He believes that a productive knowledge of words needs a connection to productive items, whereas receptive knowledge of words is not connected to any words in the lexicon; moreover, in this view no progression is seen from a receptive to a productive state (Pig - not-Shahov, 2012). However, all researchers confirm the existence of the receptive and productive dimension though they diverge in their ideas about the nature of vocabulary knowledge.

Empirical Studies on receptive / productive relationship

Exploring the relationship between receptive and productive vocabulary is called an “intriguing area” by Henriksen & Haastrup (Henriksen & Haastrup, 1998, p.77). However, this area is scarcely explored and the statements about this relationship have been vague and unsubstantiated (Laufer & Paribakht, 1998). Many research findings show that L2 learners' Receptive vocabulary size is larger than their Productive one (Laufer, 1998; Laufer & Goldstein, 2004; Laufer & Paribakht, 1998; Webb, 2008). It is assumed that gains in Receptive vocabulary knowledge often appear before Productive development (Melka, 1997).

However, there is no conclusive demonstration of the nature of the relationship between receptive and productive vocabulary knowledge, whether the gap between them is consistent or changes overtime. For instance, Wei (2007) mentioned that the results of the vocabulary test he used reflected only the students' passive knowledge and told nothing about the nature of this relationship. Also, regarding the extent of the gap between the two types of vocabulary, discrepancies have been reported. For example, it was found by Laufer (2005) that only 16% of receptive vocabulary was known productively at the 5,000 frequency level and 35% at the 2,000 level while other studies concluded that around one-half to three-quarters of receptive vocabulary is known productively (Fan, 2000; Laufer & Paribakht, 1998). In general, Nation (1990, p.48) has stated that “rough estimates indicate that learning a word productively is 50% to 100% more difficult than learning it receptively”.

Laufer (1998) as one of the forerunners in investigating this topic, compared the development of passive and active vocabulary knowledge in Grade 10 and Grade 11 Israeli EFL learners using three different types of tests i. e., “passive”, “controlled active” and “free active” tests. The results revealed that the three dimensions of lexical knowledge developed at different rates as learners proceeded in their L2 learning. Also, the progression in passive vocabulary size was more than controlled active one with an additional year of instruction while free active vocabulary did not progress at all. In 1998, Laufer and Paribakht using the same three measures examined 79 EFL, and 103 ESL learners at different proficiency levels. The results showed that the three dimensions of vocabulary knowledge developed at different rates. Active, particularly free active vocabulary, developed more slowly and less predictably than the passive vocabulary did. They also concluded that the passive/active vocabulary gap was smaller in the foreign language than in the L2 context. Fan (2000) conducted another study on 138 freshmen students in Hong Kong University. The results did not show any consistent relationship between language proficiency and the two types of vocabulary knowledge since more proficient students were found to have a larger passive vocabulary while the result was inconclusive regarding their ability to recall more of the words they can recognize.

Later, Laufer et al. (2004) researched four modalities of vocabulary knowledge using a monolingual test. Their results showed that active recall was the most difficult mode followed by passive recall and with active and passive recognition being equally the easiest modes. In a follow-up study to Laufer et al. 's (2004), Laufer and Goldstein (2004) conducted a research using a bilingual test this time with the same target word list as the monolingual test. The results showed that active recall was the most difficult mode followed by passive one, active recognition and passive recognition, and this hierarchy was seen in all frequency levels. Additionally, Zhong and Hirsh (2009) revealed a different developmental pattern in which productive vocabulary size grew faster than receptive vocabulary size after a four-month classroom instruction period among a group of intermediate Chinese students. Nemati (2010) also examined this relationship across different years of school instruction on 100 Indian ESL learners and it was found that the ratio between the two types of knowledge increased from lower to higher levels, and also despite students' progression in active and passive vocabulary knowledge, this progress was only significant for passive vocabulary not for the active one after years of instruction. Therefore, Nemati drew the conclusion that the improvement can be justified by the ESL environment not by years of instruction.

The Current Study

Every language teacher has experienced learners' understanding of lexical items when listening or reading, but not being able to produce those items in their speech or writing. Therefore, determining how much vocabulary learners know passively, and how much vocabulary learners can potentially produce would be of great pedagogical value. Moreover, vocabulary knowledge of any kind, passive or active, is strongly dependent on the educational curriculum, contexts in which learners receive and use the vocabulary, and their proficiency level. On the one hand, the question that if the proficiency levels of language learners and also the context of L2 learning has any effect on the nature of the relationship between receptive and productive vocabulary knowledge is investigated by just a few studies, and on the other hand, the inconsistent results mentioned above indicate the difficulties and confusion involved in dealing with the receptive/productive issue. So the purpose of the current study is to find out the relationship between Receptive and Productive vocabulary knowledge of Iranian EFL learners and also to observe the changes that occur in this relationship as the proficiency level of the learners increases. Here, it is intended to answer the following specific research questions:

What is the relationship between learners' Receptive and Productive vocabulary knowledge as a whole and at different frequency levels? Is this relationship similar or different at low and high proficiency levels?

Does the relationship between learners' Receptive and Productive vocabulary differ significantly across different proficiency levels?

Methodology