Friday, September 17, 2010

Stat Can Fruit & Vegetable Survey Methods Question


A Researcher is asking:

"I´m trying to gather detailed information on the Stat Can Fruit & Vegetable Survey. I have found sources stating that: Within each province, farms are stratified based into those which produce only fruit, only vegetables, or both products. A size threshold of fruit and vegetable farmland is defined; farms below the threshold ("small" farms) are randomly sampled, and a census is taken from farms above the threshold. I can´t find any information on what this "size threshold" is or how it is determined. My source of this information is here: Do you have any ideas on where else to look for this info?"

The info on Sampling, from the above web page, reads as follows. I expect the researcher is interested in the specific threshold & methodology, as applied to BC, over the years:

This is a sample survey with a cross-sectional design.

For each province, three strata were defined based on the type of operation i.e. fruit only operation, vegetable only operation or fruit and vegetable operation. In each stratum, a threshold based on the total fruit acreage and on the total vegetable acreage was defined. A random sample was selected among the operations below the threshold. A census of all operations above the threshold was taken as well. Thresholds went from 2 to 10 acres depending on the province. For each stratum, the minimum sample size was 10 and the maximum design weight was two.

Overall, the sample size was determined in order to achieve a target CV of 0.01 (1%) for total fruit area and total vegetable area at the provincial level. The final sample size for this fall survey was 9,614 operations. We excluded small farms having only two acres or less in vegetables.


Our agriculture contacts have provided the following information in response to your question below:

"The size thresholds are:
- 2 acres for Newfoundland, Prince Edward Island, Nova Scotia, and New Brunswick
- 5 acres for Alberta
- 10 acres for Quebec, Ontario, Manitoba, Saskatchewan, and British Columbia

In BC, farms with less than 10 acres of fruit and less than 10 acres of vegetables are placed in one of three sampling strata: fruit-only, vegetables-only, or both. A simple random sample is taken within each stratum, with the constraints that the sampling fraction can be no less than 0.5 and the minimum sample size is 10. A census is taken among all other farms.

The size thresholds were implemented as part of the 2008 redesign, with the goal of reducing the number of farms sent to collection while maintaining data quality. For each province, the threshold was determined so that the farms below that threshold are small enough to not contribute significantly to the estimates. Rather than sampling all of the "small" farms in a province, we sample 50% of them and assign each farm a weight of 2."

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