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[21]. *,† In Norway, there are 19 counties. However, since yield
data for Akershus/Oslo are reported together, there are 18
geographical units in this study. Annual yield was calculated
by dividing the total production of each crop per county by the
agricultural area employed in the cultivation of that crop (in
that county), and was measured in kilograms per decare.
A complete set of crop data for the years 1958-2001 for
each county was not available, most notably in northern and
western regions. In such cases, one of three approaches was
taken: where a single value was missing from a time series, it
was interpolated by calculating the average of the recordings
directly preceding and following it; where more than one con-
secutive figure for a crop was unavailable, the missing years
were removed from our analysis and the data series was broken
up into two shorter time periods; and finally, where there were
more than two consecutive breaks in the data, the entire crop
for that county was omitted from the analysis.
Weather data
The analysis required data on two climate variables impor-
tant for crop growth, namely, temperature and precipitation,
at county level in Norway. The data were obtained from the
Norwegian Meteorological Institute as retrospectively as
records permitted, allowing our period of study to extend from
2001 as far back as 1958.
The chosen parameter for temperature was GDD, which is
the annual sum of degrees accumulated above 5°C threshold. It
was calculated by aggregating the number of degrees that the
* Approximately 70% of wheat grown in Norway is sown in the spring and
the remainder is planted in the autumn. Annual and regional variations
are largely determined by weather conditions, though a general rule, win-
ter wheat production is confined to the counties of South-Eastern Norway
(Østfold, Vestfold and Akershus), where the climate is milder and thus more
suitable for crops with a high sensitivity to low temperatures.
In the period 1957-1983, the area data were based on annual sample sur-
veys, except in 1959, 1969 and 1979, when full censuses were carried out.
Since 1984, administrative sources have been used, that is, applications for
governmental production subsidies, except 1989, when a full census was
carried out. In terms of production and yield, up until the mid-1970s, the
best judgement by officials in agricultural administration at the munici-
pality level has been used. From the mid-1970s until 1989, the source has
been annual sample surveys. Since 1990, cereals production has been based
on an administrative source, that is, deliveries reported to the Norwegian
Grain/Norwegian Agricultural Authority. Potato production is still based on
annual sample surveys.
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