The statistics in this report are estimates derived from a sample. Two broad categories of error occur in such estimates: sampling and non-sampling errors. Sampling errors occur because observations are made only on samples of populations rather than entire populations. Non-sampling errors occur not only in sample surveys but also in complete censuses of entire populations. Non-sampling errors can be attributed to a number of sources: inability to obtain complete information about all sample members (e.g., some students refused to participate, or students participated but answered only certain items); ambiguous definitions; differences in interpreting questions; inability or unwillingness to give correct information; mistakes in recording or coding data; and other errors of collecting, processing, sampling, and imputing missing data. In addition, some items may be subject to more variation over time.