
Understanding the DataStudent ConfidentialityTo protect student confidentiality, individual reports have not been generated for schools where fewer than 20 students participated in the MYDAUS/YTS. When fewer than 20 students in any one grade participated, the data for that grade was not reported. Confidence IntervalsIn the Local Summary Reports bolded percentages are the weighted student response; for instance, in 2008 25.3% of the middle and high school students in Maine who answered the question said they had consumed alcohol at least once in the 30 days prior to taking the MYDAUS/YTS. However, because not all of the eligible students answered that question (or even took the survey), we have to determine statistically how well the response of the participating students might represent the response of all the eligible students in the population (in our example, the population is the State of Maine). We did this by calculating the 95% Confidence Interval (CI), meaning that if all the eligible students in the population answered the question, we are 95% confident that the results would lie between the Lower Confidence Limit (LCL) and the Upper Confidence Limit (UCL). The higher the proportion of eligible students in the population answering the survey question, the narrower the confidence interval and the more confidence we have that the results represent the experience of the student population as a whole. Therefore, the higher the participation rate at your school or district, the more confidence you can have in those results. Comparing Local and Maine Data: Is the Difference Statistically Significant?In the Local Summary Reports, if the Confidence Interval around the local results does not overlap the Confidence Interval around the results for Maine, then the local results are considered to be Significantly Different from that of Maine. If the local results are significantly lower, you will find a minus () under "Sig Diff", and if the local results are higher, you will find a plus (+). Please consider that in many instances Maine's substance use has been higher than national rates, so you need to decide if "lower than Maine" is good enough. (See MYDAUS 2008 Technical Report, page 25) Inconsistencies Between Reports1. In the Local Summary Reports, the participation rates include surveys removed after the Honesty Profile was run. The participation rate reports available under Additional Reports do not include these surveys, nor do any of the other data in either the Local Summary Reports or other reports available online. 2. In the 2008 Local Summary Reports, prior 30day cigarette use is based on the responses to Question 57: “During the past 30 days, on how many days did you smoke cigarettes?”. In the other reports, 30day cigarette use is based on the result of responses to Question 58: “During the past 30 days, on the days that you smoked, how many cigarettes did you smoke per day?”. Both questions should yield the same proportion of students reporting any use, but in reality slight differences have been observed. Weighting the DataThe students who participate in the MYDAUS/YTS may not represent all of the eligible students in the population, relative to age and gender. Because grade and gender are known to influence results, we correct for underrepresentation by weighting the data. For example, say there were 200 students at your high school and 100 were male and 100 were female; all 100 female students took the survey but only 50 of the male students participated. We would then give each female's response a weight of 1 and each male's response a weight of 2. The actual weighting of the MYDAUS/YTS data is considerably more complex, and is discussed in the 2008 MYDAUS Technical Report, Appendix A  Method of Weighting, page A6. The Honesty Profile  Screening Out Unusable SurveysThere are several questions in the MYDAUS/YTS that are used to detect surveys that were answered untruthfully (usually 34% of the total). These surveys were not included in the data available on the MYDAUS/YTS website, except in the participation rates table on page 1 of the Local Summary Reports. Margin of Error (20022004 Reports)In the school and school district reports for 2000, 2002 and 2004, a margin of error is given under each data table as a measure of how well the students participating in the MYDAUS(/YTS) in the school/district represents all eligible students in that school or district. It is based on the number of participating students, the total number of eligible students enrolled, the proportion of students who choose a response (a 50/50 split is assumed), and the desired confidence level (95%). The margin of error is reported as a plus or minus percentage. As an example, if a school's data has a margin of error of +5%, and 50% of the participating students picked a particular response, you can be 95% sure that between 45% (50% 5%) and 55% (50% + 5%) of the entire eligible student body would have picked that response. Therefore the smaller the margin of error the more confidence we have that the results of the survey represent the entire eligible student body. The margins of error by grade for the data for Maine, the OSA regions and the counties in 2000, 2002, and 2004 are listed in the tables below. (Note: In the Local Summary Reports for 2006, and 2008, a Confidence Interval for each data item replaces the one Margin of Error estimate given in the reports for earlier years.)
The margin of error calculations assume that each student in the population under study had an equal chance of becoming a participant, and that all participating students in that population answered any given question. 

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