IN AN EFFORT TO DEVELOP ACCURATE FORMULAS FOR PREDICTING AND CONTROLLING THE READABILITY OF LANGUAGE, FIVE BASIC PROBLEMS WERE STUDIED--(1) THE VARIOUS FEATURES OF WRITING STYLE AS LINEARLY RELATED TO COMPREHENSION DIFFICULTIES, (2) THE CHANGE OF STRENGTHS OF RELATIONSHIPS AS A FUNCTION OF READING ABILITY, (3) READABILITY PREDICTION MADE UPON SMALL LANGUAGE UNITS, (4) READABILITY ACCURACY BASED UPON USE OF LINGUISTIC ANALYSIS TECHNIQUES IN CONSTRUCTION OF FORMULAS, AND (5) A TEST OF USEFULNESS OF CERTAIN LINGUISTIC VARIABLES IN PREDICTING LANGUAGE DIFFICULTY. TWENTY PROSE PASSAGES WERE ANALYZED TO DETERMINE THE DIFFICULTIES OF 5,181 WORDS, 405 INDEPENDENT CLAUSES, AND 365 SENTENCES. SAMPLES WERE CHOSEN FROM FOUR PASSAGES IN EACH OF FIVE SUBJECT AREAS, USING THE DALE-CHALL SCALE FOR GRADES 4 THROUGH 8. FIVE CLOZE TEST FORMS FROM EACH PASSAGE WERE CONSTRUCTED BY DELETING EVERY FIFTH WORD. SUBJECTS, THE ENTIRE ENROLLMENT IN GRADES 4 THROUGH 8 OF ONE SCHOOL DISTRICT IN WASCO, CALIFORNIA, WERE MATCHED FOR FIVE FORM GROUPS. THE TESTING PERIOD LASTED OVER 11 SCHOOL DAYS. READING ABILITY WAS DETERMINED BY RESULTS ON THE STANFORD ACHIEVEMENT TEST--READING, FORM J, AND STUDIED IN RELATION TO THE INTERACTION OF FORM GROUPS ON MEASURES OF WORD, INDEPENDENT CLAUSE, SENTENCE, AND PASSAGE DIFFICULTY. EACH AREA WAS ANALYZED IN DETAIL AND TABULATED FOR STUDY IN LIGHT OF THE LINGUISTIC VARIABLES SET FOR THE STUDY. A BIBLIOGRAPHY OF 31 ITEMS IS ATTACHED. THIS ARTICLE IS PUBLISHED IN THE "READING RESEARCH QUARTERLY," VOLUME 1, SPRING 1966. (MC)