Overall response rates. The overall response rate represents the response rate to the survey taking into consideration each stage of the survey. For teachers, the overall response rate is calculated as the product of the response rate to two stages: the Teacher Listing Form and the teacher questionnaire.^{8 }The base-weighted overall response rate for public school teachers was 72.4 percent; for BIE school teachers, 71.4 percent; and for private school teachers, 65.9 percent. For the other surveys, the overall and unit response rates are the same since they have only one data collection stage.

Unit nonresponse bias analysis. Because the NCES Statistical Standards (4-4) require analysis of nonresponse bias for any survey stage with a base-weighted response rate less than 85 percent, all SASS files were evaluated for potential bias. First, the base-weighted unit response rate was calculated by state, region, or affiliation depending on the sector (public, BIE, and private respectively). If the base-weighted response rate for any state, region, or affiliation was below 85 percent, a detailed comparison of respondents to the frame population was conducted by examining the following characteristics: grade level, urbanicity, enrollment, and state/affiliation. A difference between the frame and respondent population was considered noteworthy if the difference was statistically significant and the following four conditions were met:

•

The relative difference between the frame and respondent population was greater

than 10 percent;

•

The absolute difference was greater than one percentage point;

•

The coefficient of variation was less than 15 percent; and

•

The cell for each subpopulation contained at least 30 interviews.

As shown in table B-1, the base-weighted response rate was 87.8 percent for public school districts. When response rates were calculated further by state, 16 states had rates below 85 percent. Table B-2 contains a list of the comparisons between the frame and the weighted distribution that were analyzed for potential bias, with an indication of the comparisons with evidence of potential bias.

Table B-2. Comparisons between frame distribution and base-weighted and final-weighted respondent distributions for public school districts with an indication of potential sources of bias: 2007-08

Base-weighted respondent distribution Final-weighted respondent distribution Potential source of bias New York, enrollment (2,500-5,000 students) Arizona, locale (rural) x x x x State (Nebraska) NOTE: x denotes comparisons that are a potential source of bias. SOURCE: U.S. Department of Education, National Center for Education Statistics, Schools and Staffing Survey (SASS), "Public School District Data File," 2007-08.

8 For the formula used to calculate the overall response rate, see NCES Statistical Standards (U.S. Department of Education 2003).

B-9