In 1923, Pressey and a co-worker published the first reading ease formula. They had been concerned that science textbooks in junior high school had so many technical words. They felt that teachers spent all class time explaining their meaning. They argued that their formula would help to measure and reduce the “vocabulary burden” of textbooks. Their formula, based on the Thorndike word list, took three hours to apply to a single book. It was a measure of vocabulary difficulty. Many later formulae made use of some measure of sentence complexity as well as vocabulary. These are the two main causes of text difficulty or 'readability'.