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Planting Survey Methodology

Planting Survey 2013 Methodology

The AHDB/HGCA Planting Survey is probably the most comprehensive pre-harvest planting survey in the market arena. it is also the first statistical survey to include spring plantings after the AHDB/HGCA Winter planting survey, which was introduced to mitigate the loss of the DEFRA December survey.

As with every other statistical survey, it is important that some statistical features be clarified.

This survey is unlike any other survey that AHDB/HGCA performs on a routine basis. It is called a ‘ratio-raising’ survey, because it uses a ratio (the difference between this and last year’s actual plantings or in this year’s case harvest area). Put simply, producers are asked what they intend to harvest this year and last year so the year on year percentage difference can be identified. This figure (ratio) is multiplied with last year’s actual regional figures to give an estimate of the cropping area. We effectively ‘raise’ this year’s figures from last year’s actual results. Each region and commodity has an individual ratio, which can be used to raise the figures. This methodology is a slight change on previous years due to the extreme nature of planting conditions. The change in methodology has been to avoid instances of individual or small groups of farms having a disproportionate impact on the estimates given.

Adjusting for smaller holdings – DEFRA, RERAD and the National Assembly for Wales provided the HGCA with June Census 2010 results stratified into the various holding sizes, and by commodity and region. For England we were supplied with both main and minor holding data from the 2010 June Census, whereas RERAD and NAW provided main holding data only (>5ha). The survey results have thus been adjusted to take into account these smallholdings for Scotland and Wales.

DEFRA, RERAD and the National Assembly for Wales provided AHDB-HGCA with 2012 results broken down by commodity and region.

Confidence Intervals  

For 2013 we have calculated confidence intervals for each regional estimate. The confidence interval, used alongside the raised estimate provides a range of values, either side of the estimate, within which the actual area sown lies within 95% i.e. if the survey was carried out 100 times, the true area planted would lie within the published range at least 95 times out of 100. The confidence intervals are also published on the HGCA website.

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