Data quality, concepts and methodology: Methodology and data quality

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Introduction

Throughout the publication, data in some instances have been updated based on more recent information from differing data sources. Revisions have been made to some 2010 commercial production estimates. The contents of this publication represent the most recent information available at the time of printing on selected fruits and vegetables produced in Canada. All data are prepared in consultation with representatives of Federal and Provincial Departments of Agriculture and l'Institut de la statistique du Québec. Current year data are preliminary and will be revised in subsequent publications and on CANSIM.

Current year area, production and value figures are preliminary while historical data are estimated based on more complete market information. Data are generally concerned with total production and take into account wastage, etc. With the availability of market and wastage information, the estimates more closely reflect commercial production.

Data shown in this publication are on a crop year basis. In the case of mushrooms, however, the data are on a calendar year basis. All value data are those obtained at the "farm gate" and are concerned with returns to growers. Estimates are provided in both imperial and metric units.

Production and value data are provided for the fourteen most commercially significant fruits grown in Canada. The production figures for fruit crops refer to marketed production rather than harvested production. Only vegetable crops grown commercially are included: no data are available for fruits and vegetables grown for home consumption.

Survey methodology

The major source of information in this publication is the Fall Survey of Fruits and Vegetables, conducted by Statistics Canada. This survey is a sample survey for which the data are collected by telephone. The survey estimates the area planted, area harvested, production and farm gate value for the principal fruit and vegetable crops. For fruit, bearing area is estimated as a subset of the cultivated area and is a sum of the fresh market and processing areas. Areas for the fresh market and for processing are also estimated for all fruit and vegetable crops.

Survey frame

The same frame is used for the Spring and Fall survey occasions.

The target population for the survey is all operations growing fruit or vegetables based on the 2006 Census of Agriculture. Observations on the 2011 survey frame have been extracted from the last three Spring survey occasions and the 2006 Census of Agriculture, to which have been added some operations coming from external sources and operations which have been identified by other surveys as growing fruit or vegetables.

Institutional farms, farms on Indian reservations, Hutterite colonies as well as operations from Yukon, the Northwest Territories and Nunavut were excluded from the survey. Operations producing only mushrooms, potatoes or greenhouse vegetables were excluded as well because these operations are already covered by other surveys collecting the desired information. Also excluded were small operations with less than 1 acre of fruit and less than 1 acre of vegetables. Ultimately, 20,132 operations were on the 2011 survey frame.

Sample selection

The Fall Survey of Fruits and Vegetables is a follow-up to the Spring survey. From the initial sample of 15,122 farms sampled for the Spring occasion, 9,306 farms were contacted again for the Fall occasion because inactive farms and small farms having only two acres or less in vegetables were excluded.

Data collection

The Fall Survey of Fruits and Vegetables collects information by telephone. Statistics Canada conducted the survey in November of 2011 using computer-assisted telephone interviews (CATI).

Response rates

Information was received from 86% of the sample. Non-responding units (i.e., no-contacts and refusals) are dealt with by adjusting the initial sample weights. The assumption that the non-responding units are randomly distributed among the sample is used to justify adjusting the initial sample weights.

Edit and imputation

It is possible to implement edit procedures at the time of the interview with the introduction of the CATI methodology. Computer programmed edit checks in the CATI system inform interviewers during the interview of possible data errors, which can then be corrected immediately by the interviewer and respondent. For the Fall Survey of Fruits and Vegetables, imputation was used to adjust for incomplete survey answers to the questions of production and farm gate value. The imputation used a trend analysis of the completed questionnaires to generate averages to be used to impute for the missing values.

Sampling and non-sampling errors

The statistics from the Fall Survey of Fruits and Vegetables are based on a random sample of agricultural operations and, as such, are subject to sampling and non-sampling errors.

Sampling errors arise when estimates are derived from sample data and not the entire population. These errors depend on factors such as sample size, sample design and the method of estimation. An important feature of probability sampling is that sampling errors can be measured from the sample itself.

Non-sampling errors are errors which are not related to sampling and may occur throughout the survey operation for many reasons. For example, non-response is an important source of non-sampling error. Population coverage, differences in the interpretation of questions, incorrect information from respondents, mistakes in recording, coding and processing of data are other examples of non-sampling errors.

Estimation

Relative to the Fall Survey of Fruits and Vegetables, the survey data collected are weighted within each stratum in order to produce estimates representative of the population. Analyses of the top contributors and historical comparisons as well as consultations with the Provincial Departments of Agriculture are performed before a final estimate is published.

Data quality

The estimates from the Fall Survey of Fruits and Vegetables are based on a probability sample of farming operations. The potential error introduced by sampling can be estimated from the sample itself by using a statistical measure called the coefficient of variation (CV).

Over repeated surveys, 95 times out of 100, the relative difference between a sample estimate and the value that would have been obtained from an enumeration of all farming operations would be less than twice the coefficient of variation.

For the 2011 survey, CVs at the Canada level for the published data range from 1.8% to 24.0%. At the provincial level, the median CV among the published data is 11.8%. Coefficients of variation values for the published data are available upon request and are not included in this publication due to space limitations.

Revision policy

Data for the previous year are presented as final while data for the current year are considered preliminary. Data are presented initially in the June issue of Fruit and Vegetable Production (Catalogue no. 22-003-X) and are revised further in the February issue of the publication.

Conversion factors

In some cases data that are received by Statistics Canada are in different units of measurement from those which appear in the publication. The fruit estimates are converted at the following rates:

1 bushel of apples = 42 pounds;
1 quart of strawberries or raspberries = 1.25 pounds;
1 pint of blueberries = 0.875 pound;
1 bushel of all other tree fruits = 50 pounds (net weight).

Corn, lettuce and cucumber production estimates are converted at the following rates:

1 dozen cobs of corn = 6 pounds;
1 dozen heads of lettuce = 15 pounds;
1 dozen cucumbers = 13 pounds.

The following metric conversion factors are used in this publication:

Other sources of information

British Columbia Ministry of Agriculture and Land
The horticulture branch provides estimates of fruit and vegetable acreage, production and value, several times a year, as more complete information becomes available.

Ontario Ministry of Agriculture, Food and Rural Affairs
Conducts several surveys and provides Statistics Canada with data for both fruits and vegetables.

Institut de la statistique du Québec
Conducts several surveys and provides Statistics Canada with data for both fruits and vegetables.