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Methodology – Data Analysis
15
Data Analysis
Weed count data on field sheets were numerically coded, entered into computer files, and verified. Data were
processed in Microsoft® Excel and summary tables were produced following the standard format used in previous
Weed Survey Series Reports.
All data were weighted to account for sites not surveyed. The field weights were calculated by taking the ratio of
the expected number of fields in the irrigation district to the number of fields surveyed in the irrigation district. The
median density, mean density, mean number of species per field and percentage of weed free quadrats were
calculated for each crop, ecoregion, county and irrigation district based on the weighted data. Standard errors were
calculated for the means and percentage of weed free quadrats.
Weed data were summarized in tables using ecological, agronomic, and jurisdictional variables including, ecoregion,
ecodistrict, crop, irrigation districts and counties. A minimum of ten sites was set to allow meaningful
summarization. Where these minima were not reached, a summary table was not provided, but data were retained in
other appropriate summaries. For example, a separate summary table for sugar beets in the Moist Mixed Grassland
was not provided; however, the fields were included in the overall summary for irrigated sugar beets. Geographic
areas (ecoregions, ecodistricts, irrigation districts and municipalities) with fewer fields than required were combined
with adjacent areas.
Data were summarized using seven quantitative variables. Details for the calculation of these variables are
described elsewhere
1
Frequency
. The number of fields in which a particular species occurred, expressed as a percentage of the total
number of fields surveyed in groups such as crops, ecoregions, or ecodistricts, and in the province.
Field uniformity (all fields).
The number of quadrats in which a particular species occurred, expressed as a
percentage of all the quadrats surveyed in groups such as crops, ecoregions, or ecodistricts, and in the province (20
per field multiplied by the number of fields).
Field uniformity (occurrence fields).
The number of quadrats in which a particular species occurred, expressed as
a percentage of the number of quadrats in groups such as crops, ecoregions, or ecodistricts, and in the province for
the occurrence fields only (20 per field multiplied by the number of fields in which the species occurred).
Field density (all fields)
. A measure of the number of plants of a species counted in a square metre. The density
values for each species in a single field are averaged over all fields surveyed in groups such as crops, ecoregions, or
ecodistrict, and in the province.
Field density (occurrence fields)
. A measure of the number of plants of each species counted in a square metre.
The density values for each species in a single field are averaged over only the fields in which the weed occurred in
groups such as crops, ecoregions, or ecodistrict, and in the province.
High density.
The highest field density values recorded for a species in groups such as in groups such as crops,
ecoregions, or ecodistricts, and in the province.
Relative abundance
. A combination of the relative frequency, field uniformity (all fields) and mean field density
(all fields) values for each species.
Relative frequency
for a species (RF) is the frequency value for a species divided by the sum of frequency values
for all species, expressed as a percentage.
Relative field uniformity (all fields)
for a species (RU) is the field uniformity value for a species divided by the
sum of field uniformity values for all species, expressed as a percentage.
11
Thomas, A.G
. 1985. Weed survey system used in Saskatchewan for cereal and oilseed crops. Weed Science
33: 34-43.