The first in a series of reports by the Far West Laboratory for Educational Research and Development, this report demonstrates the positive relationship between reduced class size and pupil achievement. The researchers collected about 80 studies that yielded over 700 comparisons of the achievement of smaller and larger classes. The results showed that as class size increases, achievement decreases. For example, the difference in being taught in a class of 20 versus a class of 40 shows an advantage of 6 percentile ranks. The relationship between class size and achievement is slightly stronger at the secondary level, but it does not differ appreciably across different school subjects, levels of pupil IQ, or several other demographic features of classrooms. The report suggests that schools cannot afford the consequences of maintaining large classes all the time and must find ways to finance smaller classes for some pupils or for all pupils for part of the school day. (Author/LD)