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This appendix presents the relationship between the demographic characteristics found in the weighted survey data and the corresponding characteristics in the study population of all U.S. households. Professional survey and data collection firms make significant efforts to ensure the rigor of their methods and to produce the highest quality results. Each year, Knowledge Networks—the company that maintains the Internet/WebTV survey panel used in this analysis—strives to create a panel that is representative of the U.S. population. However, as in any survey effort, those who respond to surveys tend to be different from those who do not. In this case, the panel used for this survey may contain subjects that are receptive to the Internet/WebTV incentive-for-service tradeoff and introduce associated biases.

Weighting used in the analyses of this report is applied to account for differences between the Internet/WebTV panel and the U.S. population. If weighting was accomplished perfectly, the distribution of various demographic characteristics in the weighted survey data would be the same as the distribution of those characteristics in national Census data. For most demographic characteristics, the two distributions are quite similar. This suggests the weighted survey results are a reasonable representation of the study population. A summary of the comparisons of demographic characteristics is provided in the table below. Detailed comparisons are provided in tables presented at the end of this appendix.

Summary of Distribution Comparisons

Number of persons in household Householder/respondent age Householder/respondent gender

One 18-24 Gender

-6.7% 6.6%

+/- 1.0%

Dwelling type Own/rent Household annual income

Single-family, attached Own/rent $75,000 and over

3.5% +/- 2.2% -3.8%

Largest Difference (Absolute Value): Demographic Characteristic Survey Estimate Less Census %

The largest differences (in absolute value) between the weighted survey data and national Census data, at between six and seven percentage points, are the number of persons in the household, and the proportion of households 18-24 years of age. The difference in the proportion of households in the highest income category is the next largest, at almost four percentage points, and the number of single-family attached residences at 3.5 percentage points. The combined over-representation of 18-24 years of age households and under-representation of single-person households, are not expected to bias the survey results in any particular direction. Differences between the weighted survey data and Census data for other demographic characteristics of the population—own/rent, and gender—are all quite small, at less than about three percentage points.


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