national multiple family submetering and allocation billing program ...

epresentative of a known population. The conclusion was that the survey respondents as a
group were representative of the population of multi-family homes from which they were drawn
across the 13 study sites. This section of the report details these quality assurance statistical
analyses.
Comparison of Postcard Survey Respondents with Sample Frame
The initial sample frame was the set of all multi-family properties in each participating
study site (identified through the utility customer information system) that had more than 10
dwelling units and/or water use greater than 200 kgal in 2001. Postcard surveys were sent to all
multi-family properties meeting these basic criteria. Quality assurance tests were conducted on
entire group of respondents and on the sample of properties from each study site that returned the
postcard survey to determine whether the water use characteristics of the postcard survey
respondents were statistically similar to the population of multi-family water use accounts
provided by the utilities (sample frame). To compare the respondents with the population,
statistical tests were used to determine whether statistically significant differences existed in
water use characteristics among the two groups.
A z-test was conducted at a 95 percent confidence level to determine if there was a
statistically significant difference in the mean annual water use of respondents versus nonrespondents
or versus the population (Test 1 in Table 3.7). The most important comparison
evaluated 2001 water use for the entire sample frame (n=19,495) with 2001 water use for the
postcard survey respondents (n=7,972). The water use in these two groups was found to be
statistically similar (i.e. not different) at a 95% confidence level. This means that on the basis of
water use alone the postcard survey respondents were representative of the population of multifamily
homes found in these 13 study sites.
Statistical comparison tests (Test 1 in Table 3.7) were performed for each participating
study site. Also, if the participating utility provided the number of units from their customer
information system, additional z-tests were conducted to determine if there were statistically
significant differences between the average water use per unit (Test 2 in Table 3.7). Summary
results of these tests are presented in Table 3.7. There were no statistically significant differences
between the water use characteristics of the study groups except for Las Vegas where the
postcard survey respondents has lower average annual water use. The research team was able to
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