There is plausible evidence that textbooks have deteriorated over time. A casual comparison of 19th and 20th century middle school textbooks shows the simplification of reading, with modern books presenting simple, dull passages (see examples here). Diane Ravitch attributes poor and inaccurate textbook content to self-censorship on the part of publishers, who fear having their products dropped as a response to noisy interest groups. Risk-averse publishers and simplified texts might be symptoms of a larger trend in American education: the emergence of large school districts who set the textbook market. The number of textbook buyers (school districts) has dropped by 90% since the beginning of the 20th century and creates a situation where publishers create products that cater to a few large, politically sensitive school districts. Milton Friedman makes a similar point when he argues that fewer, larger school districts means centralized, expensive and low quality education.