Sampling Methodology for Rice Crop Cutting Pilot Survey David J. Megill ADB Statistical Consultant Ha Noi, Viet Nam November 2015

Objectives of pilot survey Test methodology for improving quality and timeliness of rice crop area and production estimates and forecasts Includes rice crop-cutting component and farmer’s response survey Predominant rice-producing province in each of 4 countries (Viet Nam, the Philippines, Lao PDR and Thailand) selected for pilot survey

Sampling Frame Sampling frame for the Crop Cutting Survey in each country is an area frame defined using satellite images that are stratified into different categories related to the concentration of rice-growing areas Primary sampling unit (PSU) is a 200 m by 200 m square “mesh” which is spatially defined on a digitized satellite image map.

Stratification Four strata defined for sampling frame (1) Rice area maps obtained by the International Rice Research Institute (IRRI) using 2015 MODIS data and from land use maps produced by the European Space Agency (ESA) in 2009 under its GLOBCOVER initiative, which provides the spatial extent of all crops growing in the selected pilot provinces 

Stratification (continued) (2) Rice areas identified from IRRI area maps (3) Crop areas identified by GLOBCOVER (4) remaining areas where presumably rice is not grown (urban, forest, water, etc.)

Distribution of Meshes by Stratum for Each Pilot Province Country/Province Lao PDR/ Savannakhet Philippines/ Nueva Ecija Thailand / Ang Thong Viet Nam / Thai Binh IRRI 4,650 157 280 589 GLOB 154,227 66,127 2,777 4,846 IRRI+GLOB 80,839 68,514 22,105 36,376 Others 322,391 77,292 34 1,815 Total 562,107 212,090 25,196 43,626

Sample size and allocation Stratified random sample of 120 meshes selected for the pilot survey in each province Random sample of reserve meshes for possible replacement for each province Sample allocation concentrates sample in strata with more rice

Allocation of Sample Meshes by Stratum for Each Pilot Province Replacements Total IRRI+GLOBCOVER 80 10 90 IRRI 20 30 GLOBCOVER 15 5 OTHER TOTAL 120 150

Actual Number of Sample Meshes Surveyed Stratum Country/Province Lao PDR/ Savannakhet Philippines/ Nueva Ecija Thailand/ Ang Thong Viet Nam/ Thai Binh IRRI+GLOBCOVER 79 75 IRRI 19 20 GLOBCOVER 15 14 OTHER 5 6 Total 118 113 120 119

Final Distribution of Sample Meshes with Rice by Province and Stratum Country/Province Lao PDR/ Savannakhet Philippines/ Nueva Ecija Thailand / Ang Thong Viet Nam / Thai Binh IRRI+GLOBCOVER 56 66 69 63 IRRI 9 7 13 2 GLOBCOVER 10 8 OTHER 1 5 Total 77 75 91

Listing operation For each sample mesh, listing of plots with at least part of the area within the boundaries of the mesh All plots where rice would be harvested that season eligible for selection at the second sampling stage

Second sampling stage At second sampling stage, 4 plots with rice selected within each sample mesh using random systematic sampling Selection table used for selecting sample plots Area of each sample plot measured with GPS

Third sampling stage At the third sampling stage, one 2.5m by 2.5m “sub-plot” is randomly selected in each sample plot for rice crop-cutting First random corner of sample plot selected, then random length selected for each direction to identify random sub-plot

Quality control and accuracy of estimates from crop cutting data Probability sampling makes it possible to calculate unbiased estimates and measure reliability (sampling error) Quality and operational control in crop cutting and measurement techniques reduce bias in the estimation of rice yield

Weighting procedures Probabilities depend on all sampling stages where: phij = overall probability of selection of the sample sub-plot in the j-th sample plot with rice within the i-th sample mesh in stratum h

Weighting procedures (continued) nh = number of sample meshes selected in stratum h Nh = total number of meshes in frame for stratum h nhi = number of sample plots selected for crop-cutting in the i-th sample mesh in stratum h (generally equal to 4) Nhi = total number of plots with rice planted this season that are at least partly located within the sample mesh

Weighting procedures (continued) ahij = total area of the j-th sample plot with rice within the i-th sample mesh in stratum h (in m2) Weight for each sub-plot

Estimation of Total Rice Production Measurement of plots with rice used for estimating area planted in rice Crop-cutting data used for estimating yield per m2 Use of ratio estimation

Farmer Recall Survey Return to sample farms corresponding to the four plots selected for crop cutting Farm operator asked about all plots planted in rice, both inside and outside the boundaries of the sample mesh Estimation of farm characteristics based on weighted segment approach Determine proportion of famer’s rice area inside mesh

Weights for Farmer’s Recall Survey data WFhij = basic weight for the sample farm with the j-th selected rice plot in the i-th sample mesh in stratum h

Weighting procedures (continued) A’hij = area of plots planted in rice inside the sample mesh for the sample farm with the j-th selected rice plot in the i-th sample mesh in stratum h Ahij = total area of all plots planted in rice for the sample farm with the j-th selected rice plot in the i-th sample mesh in stratum h