BOARD00164 ‎(2)‎

• Board Meeting Date: 5/22/2007
• Description: OWCDP Section - Presentation by the Center for Resource Conservation - Impacts and Findings of the Slow the Flow Colorado Irrigation Inspection Program
• Board Meetings - Doc Type: Memo

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Method of Analvsis
A pair of statisticians were hired to determine and carry out statistical methods and tests
that would be most appropriate and meaningful for the purpose of this analysis. Several
key factors were taken into consideration when performing the analysis. The change in
actual ET rates from year to year can vary by several inches. Therefore, if one year was
extremely dry, households would be much more likely to use more water in that year as
compared with a year that was wetter than normal. Without taking ET rates into account,
changes in water usage could be incorrectly attributed to Slow the Flow Colorado instead
of the simple change in ET requirements. Additionally, the statisticians pointed out that
without comparing water usage in relation to landscape size, water consumption data is
not meaningful because large water users may be using an appropriate amount of water
given their landscape size and water requirements.

Although performing an analysis that expresses the results in terms of gallons saved
would be desirable, the results of such an analysis would not be statistically meaningful.
In order to conduct a statistically meaningful analysis of water consumption, data would
have to be compared against a mean, and expressing results in terms of gallons saved was
ruled out as the most accurate method of expressing the effectiveness of the Slow the
Flow Colorado Program. A large variance in the number of gallons used by each
household existed in the water records. The watering habits of households that were
using extremely large amounts of water would overshadow any changes made by
households using less amounts of water.

It was determined that percent above or below (noted as +/- ) ET would be the most
accurate and effective means of comparison. Percent +/- ET takes into account the ET
rate for the year being analyzed as well as the amount of water used in relation to the
household's landscape size. Moreover, since all of the data was expressed as a ratio, the
problem of large water users overshadowing the rest of the group was considerably
reduced.

The study groups were analyzed in several different configurations for the purposes of
performing statistical tests. Each group was analyzed as a whole for the pre-inspection
year and for the post-inspection year. Each group was also broken up into two sub-
groups, which included households that watered above ET prior to the inspection and
households that watered below ET prior to the inspection. This was done to observe
whether these two groups behaved differently following the inspection since pre-
inspection behaviors were initially different.

The standard deviation was calculated for each of the abovementioned groups and was
used to describe the distribution of the data and to form a test statistic (such as at-test).
A normality test was also performed for each ofthese groups to test whether or not the
data was normally distributed. The results ofthe normality tests helped dictate which
statistical methods were most appropriate for the analysis.