Planting Survey Methodology

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Planting Survey 

The AHDB Planting Survey is the most comprehensive pre-harvest planting survey in the market arena, whose methodology is very similar to DEFRA’s June Census. It is also the first statistical survey to include spring plantings after the AHDB 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 Cereals & Oilseeds 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 plantings) to calculate the area planted. Put simply, we ask producers what they planted for harvest both this year and last year to get the year on year percentage difference. This figure (ratio) is multiplied with last year's final June Census area figures to give us an estimate of the cropping area. We effectively 'raise' this year's figures from last year's actual results. Each region, commodity, and holding size has an individual ratio, which can be used to raise the figures.

Adjusting for sample bias - this technique (frequently employed by DEFRA) reduces sampling bias on a holding level basis. Where insufficient responses to the survey have been received, for any particular holding size range/commodity/region, then the results are combined with those of the next largest holding size/commodity/region. An assumption is made that cropping patterns will be similar across similar holding sizes for any one particular commodity and region. The original 'ratio' is thus reduced, more in line with other holding sizes. This technique is important for small surveys using 'ratio-raising' methods as extremes of responses can be minimized. This technique is used most frequently in areas with few respondents and with less popular crops, in practice usually with linseed, oats, peas and beans. This method can be employed because only the total area for each holding range is used, not individual data and the ratio (% difference between last year’s plantings and this year’s) is the key factor not the absolute value.

Adjusting for smaller holdings – RERAD and the National Assembly for Wales provide AHDB Cereals & Oilseeds with the June Census results stratified into the various holding sizes, and by commodity and region. RERAD provide main holding data only (>5ha) thus, the survey results are adjusted to take into account these smallholdings for Scotland. On the other hand, DEFRA provide both main and minor holding data from their June Census. We then apply the proportion of the different English region size bands to the DEFRA June Census area. 

Confidence Intervals

We calculate 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% of the time 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.