City Services Satisfaction Survey 2008 City Services Satisfaction Survey | Page 107

Statistical Tables
The statistical tables are labeled across the top (i.e., the banner) with the respective
cross-tabulation descriptors (bannerpoints such as male, female, Western Washington,
Balance of West). Below these Bannerpoint descriptors are the bases (the number of
respondents) used to calculate the columns of percentages. Columns of percentages
which add to more than 100% are the result of computer-rounding errors or multiple
responses. Small differences from 100% are usually computer-rounding errors, while
large differences typically are the result of multiple responses.
Statistical Variation. All percentages shown in the statistical tables are subject to
statistical variation, or statistical error. The smaller the sample of respondents (i.e., the
smaller the "base"), the larger is the statistical variation in the corresponding
percentages, usually. The table below presents approximate statistical error for
percentages based upon different sample sizes.
Statistical Error Ranges*
Size of
Sample 50%
50
75
100
150
200
250
300
400
500
600
700
800
900
1000 ±14.0
±11.4
±9.8
±8.0
±6.9
±6.2
±5.6
±4.9
±4.3
±4.0
±3.7
±3.4
±3.2
±3.0
At Various Percentage Levels
40%
30%
20%
or
or
or
60%
70%
80%
±13.7
±11.1
±9.6
±7.8
±6.8
±6.1
±5.5
±4.8
±4.3
±3.9
±3.6
±3.3
±3.1
±3.0
±12.8
±10.4
±9.0
±7.3
±6.3
±5.7
±5.2
±4.5
±4.0
±3.6
±3.3
±3.1
±2.9
±2.8
±11.2
±9.1
±7.9
±6.4
±5.5
±4.9
±4.5
±3.9
±3.5
±3.2
±2.9
±2.7
±2.6
±2.4
10%
or
90%
±8.3
±6.8
±5.9
±4.8
±4.2
±3.7
±3.4
±2.9
±2.6
±2.4
±2.2
±2.0
±1.9
±1.8
*At 95% level of confidence.
When sample sizes are small (less than 100), extra caution should be exercised in
interpreting the corresponding percentages.
Decision Analyst, Inc.
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