4. Methodology and Quality Note
This section provides a summary of information on these statistics against five dimensions of quality, based on the European Statistical System (ESS) quality framework: Relevance, Accuracy, Timeliness and Punctuality, Accessibility and Clarity, and Comparability. The Scottish Government adheres to the Code of Practice for Official Statistics and the National Statistician's guidance on quality. In addition the Scottish Government provides its own guidance on quality, which is available to view at the Scottish Government's Statistics internet pages.
Links to Guidance on Quality:
- Code of Practice for Official Statistics
- National Statistician's Guidance on Quality
- Scottish Government's Corporate Policy Statement
- Scottish Government Guide to basic quality assurance
- European Statistics Code of Practice (including quality framework)
At the end of this section is an estimate of the total financial cost to respondents of participating in the Cereal Production Survey (CPS), which provides the data that this release is based on.
4.1 The relationship between area, yield and production
Cereal and oilseed rape crop areas represent the amount of area that has been planted with a particular crop, which is intended for combine harvesting and the production of grain or oilseeds. These area estimates are derived from the June Agricultural Census and specifically exclude any areas of cereals which are not intended for combine harvesting. Whole crop cereals are harvested whole (ie without extracting the grain) and used as source of animal feed.
Average yields are expressed in tonnes per hectare and represents the amount of cereal grain or oilseed that is extracted from 1 hectare of combine harvested area. As the moisture content of cereals and oilseeds can vary from year to year, depending on the level of rainfall, average yields are adjusted to a standard moisture content of 14.5% for cereals and 9% for oilseeds. This adjustment ensures there is consistency in estimates of the amount of dry matter which can be extracted from cereal grain and oilseeds.
Production estimates are derived by multiplying crop areas (in hectares) and average yields (in tonnes per hectare). They represent the total tonnage of cereal grain and oilseed that is combine harvested from the planted area. This tonnage does not include the weight of straw and other plant material which is produced as a by-product and utilised for other purposes.
The 2012 estimates of production are based mainly on final yield results from the 2012 Cereal Production Survey (CPS) and final crop areas from the 2012 June Census. The CPS is a disproportionate stratified random sample of around 400 farms in Scotland stratified by region. The construction of the sample is based on the Neyman Allocation which apportions larger sample sizes to the strata with the most variation in yields.
In previous years (results prior to 2012) the CPS sample was stratified by region and size group. To help ensure sufficient sample sizes for most crop types stratification by size group was replaced by crop type in 2012.
In 2012, the number of holdings submitting a return for Spring Barley was 366, Winter Barley was 78, Wheat was 150, Oats was 45 and Oilseed Rape was 42. For some regions relatively few returns were received for some crops.
Totals of sample production and sample crop area for each stratum (i.e. crop and region combination) are used to derive a sample estimate of yield. These yield values are applied to national crop areas from the June Agricultural Census to provide national estimates of production. Where sample sizes for strata are insufficient to calculate production results national average yield estimates for the crop are used to calculate estimates of production.
2012 regional results were based on national averages for: spring oilseed rape in all regions; winter oats in the North East, North West and South West; winter oilseed rape in the South West; and winter barley in the South West. Estimates of Triticale production are based on estimates from the annual Crop Report Meeting (CRM).
The Cereal Production Survey is carried out by Rural Payments & Inspections Division (RPID) and Rural and Environment Science & Analytical Services (RESAS) within the Scottish Government (SG). The survey is carried out by telephone with forms mailed to farms on request. Completed returns are analysed by RESAS.
The data undergo several validation processes as follows; (i) checking for any obvious errors on the paper survey forms upon receipt, (ii) cross checking against Census area data and internal validation within survey forms to ensure totals match, (iii) results are standardised to 14.5 per cent moisture content for cereals and 9 per cent moisture content for oilseed rape (iv) assessing data for any extreme yield values and removing if necessary, (v) if required, area offices are contacted to ensure that data is correct.
Additional quality assurance is provided at the later stages by utilising expert knowledge within the Scottish Government.
Data quality and assurance measures used for June Census area data are contained in Final Results From The 2012 June Agricultural Census.
Provisional Estimates - published on 15th October 2012
The provisional estimates are derived from yield values of individual growers collated by several industry bodies. These industry bodies meet to discuss and quality assure these estimates at the annual Crop Report Meeting (CRM), which in 2012 was attended by representatives from:
- Scottish Government, Analytical Services
- Rural Payments and Inspections Directorate
- CFG Masstock
- Baird Malt Ltd
- Scottish Agricultural College (Now SRUC)
- Agricultural Industries Confederation
- NFU Scotland
- Highland Grain Ltd
- The Agriculture and Horticulture Development Board (which now includes the Home Grown Cereals Authority)
Estimates from growers are collected by several means, either: through area offices of the Scottish Government (SG) Rural Payments and Inspections Directorate (RPID) or area offices of the Scottish Agricultural College (SAC); from agronomists working for commercial bodies; through farming co-operatives; or through the National Farmers Union (NFU), using electronic, paper based or telephone surveys.
Average yields from known harvested areas are collected from all regions in Scotland for each individual crop. For consistency, these average yields are adjusted to a standard moisture content of 14.5% for cereals and 9% for oilseed rape.
Once all the yields have been collated, the industry bodies at the Crop Report Meeting carry out additional quality assurance by comparing resulting yields between different crops and regions within Scotland. This results in an agreed set of yield estimates which are then combined with June Agricultural Census area results to derive the harvest production estimates.
More information on the methodology and results of the 2012 first estimates of the cereal and oilseed rape harvest can be found in the first estimates of the cereal and oilseed rape harvest release.
The accuracy of the 2012 first estimates (compared with the final estimates presented here) is assessed in section 4.35.
The degree to which the statistical product meets user needs for both coverage and content.
The cereal estimates are produced for a wide range of purposes. The statistics help the government to form, monitor and evaluate policy, and to assess the economic well-being of the cereal sector. They are also required by law by the Statistical Office of the European Communities, as the information is essential for management of the EU markets. The provisional estimates (published on 15/10/2012) are timed to enable provision of data for an EU regulatory deadline. Specific regulations are listed on pages 3 to 5 of our 2009/10 annual statistics plan; a link is provided here;
An updated plan will be available before April 2013.
The production estimates presented here also feed into the UK cereals balance sheet, which provides an independent, unbiased, timely and comprehensive picture of the supply and demand position of the UK cereal market. The balance sheet is also the prime tool for tracking new developments in the UK cereals industry and determining their impact on the market. The balance sheet is widely used by policy makers, the EU Commission and the wider cereals industry. A link to the balance sheets, published by the Home Grown Cereals Authority (HGCA), is provided here;
Though we are not aware of any unmet user needs in relation to these statistics, the Scottish Government is always interested to here from users about what is most relevant to them and welcomes feedback from users of these statistics. Contact details are available from the Agriculture Statistics contacts webpage, http://www.scotland.gov.uk/Topics/Statistics/Browse/Agriculture-Fisheries/Contacts.
Details of both current and past user consultations are available on the Agriculture Statistics consultations webpage, http://www.scotland.gov.uk/Topics/Statistics/Browse/Agriculture-Fisheries/scotstat.
The closeness between an estimated result and the (unknown) true value.
The number of agricultural holdings surveyed in the CPS was (445) in 2012. Usable returns were received for 422 of these; a response rate of 95%. Although 445 holdings were surveyed, many holdings grow more than one crop. The total number of returns received for all crops combined was 684, this equates to a sampling rate of 5% overall. The 2012 CPS sample covered 5% of the relevant planted areas in Scotland.
The results from the CPS have a margin of error associated with them, reflecting the error resulting from sampling. Sampling error is the difference between the estimate derived from a sample survey and the true value that would result if a census of the whole population were taken under the same conditions.
The sampling error can be estimated and used to produce confidence intervals around the survey results. These intervals tell us the range of values within which the true value lies, with a given degree of confidence. The intervals below are 95% confidence intervals; this means that if the sample survey was repeated a large number of times, 95% of the resulting estimates would lie within the intervals around our sample estimates. For example, there is a 95% chance that the true production value for all cereals in Scotland will lie within the range of 2.5 million tonnes ±117,000 tonnes. Charts A and B, below, show the main production estimates marked with the upper and lower bounds of the associated confidence intervals. This is shown on two charts with different scales to allow results to be viewed clearly.
Table A - 95% Confidence Intervals for 2012 CPS Estimates
|Crop||Number of Holdings (June Census)||Sample Size||Sampling %||Production ('000 tonnes)||Confidence Limits ('000 tonnes)||Confidence Limits (%)|
1 includes Triticale, excludes Oilseed Rape
Charts A and B - Production Estimates with Confidence Intervals: 2012
Area data are sourced from the June Agricultural Census and are assumed to be accurate as farmers face financial penalties for supplying incorrect information.
In previous years (results prior to 2012) production estimates were based on the mean average of individual yield values for each of the strata. This has been changed to provide more reliable results (see methodology section). Results have been revised from 2003 onwards. At this time it is not possible to revise results prior to 2003. Charts C to H, below, show the impact of the revisions on the production estimates for the main crop groups. Note that the vertical axes of the charts do not start at zero; this is to highlight the variation between years and the differences between results of original and revised estimates.
Charts C to H - Comparison of Original and Revised Production Estimates
In 2012, the revised estimate of total cereal production was 172,000 tonnes (7%) higher than the original estimate. Since 2003 the difference between the resulting estimates of the two methods has fluctuated between 1% and 7%. In 2012 the new revised estimates result in differences of: +147,000 tonnes (up 11%) for spring barley; +9,000 tonnes (up 3%) for winter barley; +14,000 tonnes (up 2%) for wheat; +2,000 tonnes (up 2%) for oats; and +2,000 tonnes (up 2%) for oilseed rape.
4.33 Timeliness and Punctuality
Timeliness refers to the lapse of time between publication and the period to which the data refer.
To provide reliable estimates of the year on year changes in production, the CPS is carried out at the same time each year. The reference date for the CPS, the date at which respondents are asked for production information, is the 31st October each year. Typically, at the end of October the vast majority of the Scottish cereal and oilseed rape harvest is complete, allowing for reliable estimates to be made.
The release of results is completed within two months of this date, to allow sufficient time for data collection, processing, quality assurance and compilation and dissemination of final results. The specific date of release is chosen to coincide with the release of DEFRA's equivalent estimates as Scottish results feed into UK estimates and UK cereals balance sheet.
To help inform users, both those within and outwith the agricultural industry, early (provisional) estimates are provided, based on the results of the annual Crop Report Meeting (CRM). In order to provide timely estimates of cereal and oilseed rape production the CRM is held as early as is practical following data collection by the industry representatives attending the meeting. The progress of the harvest limits how early this meeting can be held. In 2012, due to poor weather, Scotland experienced a late harvest. To allow further harvest progress prior to data collection the Crop Report Meeting was held on the 9th October, when the majority of the Scottish harvest had been completed, the results were published under National Statistics protocols on the 15th October.
Punctuality refers to the time lag between the actual and planned dates of publication.
The results of the 2012 CPS were released on the scheduled date of 20th December 2012.
4.34 Accessibility and Clarity
Accessibility is the ease with which users are able to access the data. It also relates to the format(s) in which the data are available and the availability of supporting information.
Clarity refers to the quality and sufficiency of the metadata, illustrations and accompanying advice.
These statistics are made available online at the Scottish Government's statistics website in accessible formats (html and pdf versions are available). All data tables are made available in excel format to allow users to carry out further analysis. Methodological notes and additional notes to tables, identifying specific quality issues, are included in this document, which is available online and linked to from all National Statistics outputs containing cereal production estimates. Links to the Agriculture Statistics series of outputs are available at the following internet address, http://www.scotland.gov.uk/Topics/Statistics/Browse/Agriculture-Fisheries/Publications.
The degree to which data can be compared over time and domain.
Results for England, Wales and Northern Ireland are compiled on a comparable basis with Scottish estimates. The latest UK results can be accessed at the following internet address, http://www.defra.gov.uk/statistics/foodfarm/food/cereals/cerealsoilseed/.
The EC regularly produces estimates of cereal and oilseed production both EU-27 countries and individual countries. Further information on EC cereal statistics is available at the following website:
http://epp.eurostat.ec.europa.eu/statistics_explained/index.php/Agricultural_products Typically EC results are published later than Scottish or UK results due to the additional time required to collate, validate and analyse data from several countries. Users interested in comparing results between countries should evaluate the relevant methodologies of sources used.
Comparison of provisional and final results (Table 3)
This section compares past provisional estimates of the harvest to the final estimates of the harvest. Provisional estimates are derived from averaged yield estimates of growers, collated through the cooperation of several organisations within the agricultural sector, applied to crop area estimates from the June Agricultural Census. Final estimates are derived from average yields from the Cereal Production Survey (CPS).
Although provisional harvest estimates are not based on statistical methods, they are representative of about 24% of Scottish cereal production and can be used to compare with final estimates. If there were large differences in the estimated values between the two sources this would suggest that, at least, one of the sources provided a poor estimate of actual production.
Prior to 2009, the June Agricultural Census issued two sets of results (provisional and final). This meant that area figures used to calculate the provisional production estimates were slightly different from the final June Agricultural Census areas used to calculate the final production estimates. However, these differences were generally small (less than 1 percent) and were not a main contributor to differences in the production estimates. Since 2009, administrative data on land use from the Single Application Form (SAF) has replaced land data collected through the June census. This has meant that only one set of final results are now issued from the June Agricultural Census - meaning that consistent area figures are available for the provisional and final harvest estimates.
The main reason for differences in the provisional and final production estimates are differences in provisional and final yield estimates. Chart I shows these differences for the last 10 years. It can be seen from Chart I that for the last 10 years the provisional estimate of the total cereal harvest has been within around 5% of the final estimate. In most years, the largest differences between provisional and final production estimates are for oats, with the largest difference being 17% in 2012.
Chart I: Percentage Difference in Cereal Yield (Final v Provisional Estimates): 2003 to 2012
As the results of the provisional and final estimates are similar, this provides additional confidence in the results of the cereal production survey. Estimated production for oats varies considerably between the provisional and final estimates. With the exception of oats, the 2012 provisional estimates all fall within the confidence intervals of the final estimates from the Cereal Production Survey.
4.36 Respondent Burden (the estimated overall cost to respondents)
The estimated respondent burden is calculated as the total number of survey responses (A), multiplied by the median time taken to respond to the survey (B), multiplied by the median average hourly wage of typical respondents (C).
(A x B x C)
(A) The 2012 Cereal Production Survey (CPS) received 422 responses.
The time taken to respond to the survey varies with each respondent. Scottish Government (SG) Rural Payments and Inspections Directorate (RPID) staff conducting the survey were asked to provide estimates of the average time taken to administer the telephone survey. Estimates were collected from each area office involved in the survey and the median time to respond in hours was calculated from these responses.
(B) The median time taken to respond to the survey is 0.083 hours.
Respondents to the CPS are usually farm owners themselves or farm managers. Both are usually full-time workers.
(C) The estimated median hourly pay rate for full-time employees in Scotland in 2012 was £12.52
The respondent burden for CPS data collection in 2012 was
422 X 0.083 X £12.52
4.37 Related publications
Results from all Scottish Government agricultural surveys can be accessed here:
Email: Andrew Walker
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