BACKGROUND

Text Inspector helps you to analyse the vocabulary content of texts, particularly for second language learners. Why vocabulary?
Take reading for example. In an influential article from 2010, which you can find here, Laufer and Ravenhorst-Kalovski note that their large-scale research project
“corroborates once more the earlier claims that vocabulary may be the major factor in reading comprehension” (Laufer & Ravenhorst-Kalovski 2010:26, emphasis added).
Why is this? Because they found that “64% of variance in the reading score is accounted for by vocabulary” (idem).

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This was confirmed in Milton’s study of vocabulary in terms of the Common European Framework, in which he stated that for all but one language analysed, “some 60 to 70% of variance in CEFR levels can be explained by differences in vocabulary size” (Milton 2010:211)
Other important studies related to reading are summarised by van Zeeland (2013)
“One much-cited reading study was carried out by Hu and Nation (2000). They presented L2 readers with a short text with varying degrees of pseudo-words and investigated how this affected their text comprehension. The results suggested the required coverage for reading comprehension to be around 98 percent. More recently, Schmitt, Jiang and Grabe (2011) explored L2 readers’ comprehension at each percentage point of coverage between 90 percent and 100 percent, which revealed a linear coverage–comprehension relationship. This suggests that the lexical coverage required depends on the comprehension level aimed for.” (van Zeeland 2013:2)
In other words, vocabulary knowledge is clearly the most important dimension in reading. However, we must not forget the other factors. Reading difficulty comes also from the grammar, content, world knowledge and other sources.
In terms of listening, the picture is similar, although probably other factors besides vocabulary play a bigger part. van Zeeland summarises the position as follows (and see the full text for details):
“The few studies that have been carried out in this realm have indicated that vocabulary knowledge is an important factor in L2 listening ability. ……….Overall, then, research shows that vocabulary knowledge is an important predictor of L2 listening ability, but also suggests that a wide variety of factors besides vocabulary knowledge affect listening ability.” (van Zeeland 2013:1)
As for writingStæhr‘s study concluded that:
“As regards the productive skill of writing, this was found to correlate significantly and fairly highly with vocabulary size (0.73). In addition, more than half of the variance in the ability to perform above average in the writing test was explained by vocabulary size”. (Stæhr 2008:148)
In speaking too vocabulary is seen as crucial, maybe even the most crucial element. Koizumi summarises the picture by saying that:
“Vocabulary plays an essential role in oral production. In theoretical models of first and second language (L1 and L2, respectively) speaking processes ……vocabulary is indispensable in producing speech with appropriate meanings as well as in generating syntactic, morphological, and phonological structures.
Empirical studies on vocabulary and speaking proficiency are limited in scope and smaller in number than those on vocabulary and reading proficiency. However, some studies have shown the greater importance of vocabulary in speaking proficiency compared to other linguistic elements such as pronunciation and grammar.” (Koizumi 2013:1) 

What does this mean for Text Inspector?

We need to take account of all of this when we use Text Inspector for analysing texts of all types and sources. It is important not to claim that the results we obtain when analysing the lexis of textx via Text Inspector are the whole story.
However, the vocabulary content of a text is certainly a very important dimension – and this is broadly speaking as true of writing, speaking and listening as it is of reading – which is why Text Inspector helps you to analyse it.
References
Hu, M., & Nation, I. S. P. (2000). Unknown vocabulary density and reading comprehension. Reading in a Foreign Language, 13(1), 403–30.
Koizumi, R. 2013. Vocabulary and Speaking. The Encyclopedia of Applied Linguistics. 1–7.
Laufer, B. & G. Ravenhorst-Kalovski (2010) ‘Lexical threshold revisited: Lexical text coverage, learners’ vocabulary size and reading comprehension’ Reading in a Foreign Language, ISSN 1539-0578, April 2010, Volume 22, No. 1,pp.15-30
Milton, J. (2010) The development of vocabulary breadth across the CEFR levels, in The development of vocabulary breadth across the CEFR levels, I. Bartning, Martin, M., and Vedder, I. Eurosla, 2010, pp. 211-232
Schmitt, N., Jiang, X., & Grabe, W. (2011). The percentage of words known in a text and reading comprehension. The Modern Language Journal, 95(1), 26–43.
Stæhr, L. S. (2008). Vocabulary size and the skills of listening, reading and writing. Language Learning Journal, 36(2), 13952
van Zeeland, H. (2013) Vocabulary and Listening. The Encyclopedia of Applied Linguistics. 1–6.