**Summary:** To create a model for teacher need, we estimated a replacement rate. Using the current population of high school students in the United States and an assumption about how many students will enroll in physics, we estimated the number of physics teachers required. We then multiplied that number by a published rate of teacher attrition, resulting in the need for new teachers each year. This estimated need highlights the inadequate numbers of new physics teachers actually prepared across the nation. We note that this estimate is intentionally conservative. Other researchers have looked at the shortage of certified STEM teachers in the United States, but for this analysis, we focused on comparing the rate at which new physics teachers are prepared versus the rate at which current teachers leave the field due to retirement or other causes of attrition.

We have also estimated what this need would be if *all* high school students took physics, which is a long-term goal of many educators. To simplify the calculation, we assumed that each student in a school that includes grades 9-12 will take one physics course prior to graduation.

## Calculations, assumptions, and sources of data

The total number of high school physics teachers required was based on the number of students enrolled in public and private high schools in the US, including the 50 states, the District of Columbia, and Puerto Rico. These data are publicly available from the NCES ElSi. Since schools offer physics at different grades (e.g., as a senior class, or as a "Physics First" 9th grade class), we take the average number of students present in grades 9-12 in a given school to be the number of students eligible to take physics in a given academic year. We then applied the assumed physics enrollment rate to obtain the estimate of how many students in that school will take physics in a given academic year. The NRC’s main figures use an assumption that 40% of high school students will take physics prior to graduation, based on data from the American Institute of Physics and the NCES for the national average of the fraction of high school students who take a physics course.

We then made an assumption about the minimum class size necessary for a school to support having at least one physics course (e.g., if only a single student were to take physics, the school may not have the resources to offer the course). This minimum class size was estimated using publicly available data from the Office of Civil Rights at the Department of Education (CRDC survey data), which includes the number of physics classes and the number of students enrolled in physics in each public school in the US. For each state, we estimated the 20th percentile of the distribution of the average physics class sizes in the state’s public schools. This value was used as the minimum number of physics students required to support at least one physics class in any school in that state. Because comparable data is not available for private schools, we also applied this minimum for each private school in a state to have at least one physics class. Note that in the case where we assume all students take physics (100% enrollment), a minimum class size is not applied to the analysis.

To generate a conservative estimate of teacher need, we then assumed (1) that a single physics teacher can serve a maximum of 5 class sections in a school and (2) that each class section can have a maximum of 30 students, based on an average across states with legal caps on class size. This approach resulted in an estimate of about ~27,700 physics teachers in the United States, which is close to the total number of ~27,600 physics teachers reported by the AIP Statistical Research Center.

To determine how many new physics teachers are needed each year, we assumed that 7% of physics teachers would leave the profession each year (see the "Teacher Attrition Rate" section below). Thus, the estimated total number of new physics teachers needed each year in the U.S. is about 2,000. This is greater than the number of new physics teachers actually hired, which was estimated at about 1,400 according to the Task Force on Teacher Education in Physics (T-TEP). T-TEP also found that teacher numbers were augmented by in-service teachers of other subjects who got assigned to physics.

Our analysis had additional nuances for the case where all (100%) of high school students take physics prior to graduation. To reach this scenario, additional teachers would need to be hired to meet the increased enrollment versus the current state of physics teaching. However, our model did not account for these additional hires and maintains the analysis as a replacement rate. For this scenario, our analysis answered the question "if we had enough qualified teachers so that all students can take physics prior to graduation, how many new teachers would be needed each year to replace those who leave due to retirement or other means of attrition."

Finally, we also note that this calculation differs from what has been done in prior versions of the NRC. In short, the methods differ in how (and whether) they apply assumptions about class sizes and physics enrollment rates. For more details or specific questions, please contact us.

### Teacher attrition rate

To calculate the level of need for teachers, we used a teacher attrition rate of 7.0%. This value is from the *Attrition of Public School Mathematics and Science Teachers*, a 2008 issue brief from the National Center for Education Statistics, which reported that STEM teacher attrition is lower than the overall secondary teacher attrition rate.

Other related estimates of teacher attrition rates were considered but not used, including

- The American Institute of Physics's Statistical Research Center estimated the attrition rate of physics teachers is 4%, based on its 2010 survey of in-service physics teachers.
- A 2009 report published by the Consortium for Policy Research in Education used data from the Census Bureau’s Schools and Staffing Survey reports to identify the numbers of teachers in different subjects (English, Science, etc.) entering and leaving the profession each year. In the general subject area of Science, the ratio of the number of teachers who left the profession to the total number in the workforce gave an estimated attrition rate of 9.69%.