national multiple family submetering and allocation billing program ...

hybrid properties appeared to use substantially less water than either in-rent or RUBS properties,
but the sample size of these properties was, again, too small to be considered conclusive.
Table 5.8 2001 and 2002 annual indoor per unit water use - manager survey respondents
In-rent or
HOA Submetered RUBS
Hot water
hybrid
Average (kgal) 51.56 46.20 54.85 47.17
2001
Std Dev (kgal) 25.43 23.65 23.72 37.34
Median (kgal) 46.75 42.44 52.19 40.72
Number N=847 N=113 N=175 N=22
Average (kgal) 52.60 47.04 52.95 43.33
2002
Std Dev (kgal) 27.91 22.35 23.98 38.06
Median (kgal) 46.92 44.09 49.55 32.87
Number N=774 N=112 N=169 N=17
Four separate t-tests were performed comparing the 2001 in-rent water use against the
2002 in-rent water use, the 2001 submetered water use against the 2002 submetered water use,
and so on. Using an alpha value of 0.05 that corresponds to a 95% confidence interval, it was
found that there was no statistically significant difference between the water use from these two
years. Because of the increased statistical power of a larger dataset, annual indoor per unit water
use data for 2001 and 2002 from the manager survey respondents were combined by averaging
the water use from each year. If water data were available from only one of the years, that
estimate was used.
For most of the analyses in this report, water use was normalized by number of units,
however, it is interesting to look at the results at the more basic level of total indoor water use
verses number of units for each property. Also, it is very helpful to look at plots that show all of
the data in addition to the best fit lines so that the fit of the model and the scatter of the data can
be seen by the reader.
Figure 5.4 shows the relationship between the total indoor water use and the number of
units by billing type for the manager survey respondents. Separate trendlines were fit to the data
for in-rent, RUBS and submetered properties. The trendlines were plotted as power curves in
order to determine their linearity. It is striking that in each case the exponent is very close to 1,
which confirms that there is a linear relationship between the number of units on a property and
its total indoor use. This also provides justification for normalizing the water use on the basis of
the number of units. In Figure 5.5, a similar graph was developed for just those properties built
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