The 600 words in each book of this series along with the additional target words
presented in the appendices included in the first three books of the series are
the most useful words in English. They were found by analysis of a collection of
English course books from various levels in the primary, secondary and tertiary
school systems. The words included in this series were chosen because they
occurred many times in different levels of these materials. Because of the way
that they were chosen, these words have the following characteristics: