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BACK in 1922, Thomas Edison predicted that "the motion picture is destined to revolutionize our educational system and...in a few years it will supplant largely, if not entirely, the use of textbooks." Well, we all make mistakes. But at least Edison did not squander vast quantities of public money on installing cinema screens in schools around the country. With computers, the story has been different. Many governments have packed them into schools, convinced that their presence would improve the pace and efficiency of learning. Large numbers of studies, some more academically respectable than others, have purported to show that computers help children to learn. Now, however, a study that compares classes with computers against similar classes without them casts doubt on that view. In the current Economic Journal, Joshua Angrist of the Massachusetts Institute of Technology and Victor Lavy of the Hebrew University of Jerusalem look at a scheme which put computers into many of Israel's primary and middle schools in the mid-1990s. Dr Angrist and Dr Lavy compare the test scores for maths and Hebrew achieved by children in the fourth and eighth grades (ie, aged about nine and 13) in schools with and without computers. They also asked the classes' teachers how they used various teaching materials, such as Xeroxed worksheets and, of course 更多：https://www.bmcx.com/ , computer programs. The researchers found that the Israeli scheme had much less effect on teaching methods in middle schools than in elementary schools. It also found no evidence that the use of computers improved children's test scores. In fact, it found the reverse. In the case of the maths scores of fourth-graders, there was a consistently negative relationship between computer use and test scores. The authors offer three possible explanations of why this might be. First, the introduction of computers into classrooms might have gobbled up cash that would otherwise have paid for other aspects of education. But that is unlikely in this case since the money for the programme came from the national lottery, and the study found no significant change in teaching resources, methods or training in schools that acquired computers through the scheme. A second possibility is that the transition to using computers in instruction takes time to have an effect. Maybe, say the authors, but the schools surveyed had been using the scheme's computers for a full school year. That was enough for the new computers to have had a large (and apparently malign) influence on fourth-grade maths scores. The third explanation is the simplest: that the use of computers in teaching is no better (and perhaps worse) than other teaching methods. The bottom line, says Dr Angrist, is that "the costs are clear-cut and the benefits are murky." The burden of proof now lies with the promoters of classroom computers. And the only reliable way to make their case is, surely, to conduct a proper study, with children randomly allocated to teachers who use computers and teachers who use other methods, including the cheapest of all: chalk and talk.