Five reports from a 2-year study are presented. Frequencies and descriptions of systematic errors in the four algorithms in arithmetic were studied in upper-middle income, regular, and special education classrooms involving 744 children. Children were screened for adequate knowledge of basic facts and for receiving prior instruction on the computational process. Systematic errors contain a recurring incorrect computational process and are differentiated from careless errors and random errors. Errors were studied within levels of computational skill for each algorithm. Results showed that five to six percent of the children made systematic errors in the addition, multiplication, and division algorithms. The figure was 13 percent for the subtraction algortihm. One year later 23 percent of the children were making either the identical systematic error or another systematic error. (Author/JP)