One way to do this is to use a set of graded test passages. There the correlation coefficient of the better formulae "hovers around 70%". There have been many dozens of experimental tests, summarised by Klare. Correlations between readability measures and comprehension scores on passages are usual, as are correlations between readability scores are grade levels as chosen by experienced teachers. Quite an important result was got by Murphy, who increased the readability of a farm journal and found the readership much increased.