The Editorial Projects in Education Research Center uses the Cumulative Promotion Index (CPI) method to calculate graduation rates. The CPI represents the high school experience as a process rather than a single event, capturing the four key steps a student must take in order to graduate: three grade-to-grade promotions (9 to 10, 10 to 11, and 11 to 12) and ultimately earning a diploma (grade 12 to graduation). Each of these individual components corresponds to a grade-promotion ratio.

The equation below illustrates the CPI formula for calculating graduation rates. The class of 2009-10, the most recent year of data available, is used as an example.

Multiplying the four grade-specific promotion ratios together produces the graduation rate, the percent of public school 9th graders who will complete high school on time with a regular diploma. The CPI counts only students receiving standard high school diplomas as graduates, following the definition of a graduate established by the No Child Left Behind Act.

We can use a simplified example to further demonstrate how the center calculates the CPI. Let us suppose that a particular school district currently has 100 students enrolled in each grade from 9 through 12. We will also assume that 5 percent of students currently in grades 9, 10, and 11 will drop out of school this year and that 5 percent of seniors will fail to earn a diploma at the end of the year. So, for example, we would count 100 9th graders at our starting point but only 95 10th graders the following fall.

Carrying out the calculation (shown above), we arrive at a graduation rate of 81.5 percent for this district. Given conditions in this hypothetical district (an effective 5 percent annual attrition rate for students at each grade level), only about 82 out of every 100 9th graders would be expected to finish high school with a diploma.

The CPI can be calculated for public school districts that have students enrolled in the secondary grades (9 through 12). State and national statistics are generated by aggregating the district-level data upward.

The EPE Research Center calculates graduation rates using data from the Common Core of Data (CCD), an annual census of public schools and school districts in the United States conducted by the U.S. Department of Education. Detailed methodological descriptions of the CCD can be found in technical documentation published by the National Center for Education Statistics (available online at nces.ed.gov/ccd). For the 2009-10 school year, diploma counts were not available from the CCD for Connecticut. The EPE Research Center obtained those data directly from the state education agency where available.

The center's goal is to provide a measure of the graduation rate for each of the roughly 11,000 school districts in the nation that enroll high school students. To provide fuller and more representative coverage of the student population, the center used a multivariate statistical model to impute graduation rates for certain districts where a directly calculated rate was not available.

To avoid the unintentional disclosure of information about individual students, the EPE Research Center does not report results for very small demographic subgroups, those with fewer than five students in a given category. Additional procedures are employed to ensure that results are only reported in situations where sufficient data are available for a reliable calculation.