The Dolch words are
the 220 most frequently found words in books that children read. These
words are usually learned in first and second grade; students who learn
these words have a good base for beginning reading. Many of these words
cannot be sounded out because they do not follow decoding rules, so they
must be learned as sight words.
One way of
estimating a primary student’s reading level is by having the student
identify the 220 Dolch Basic Sight Words. The number of words recognized
is the basis for assigning his/her equivalent reading level.
http://www.englishzone.com/reading/dolch.html