SE-ASIA-opium-poppy-2014-web

Regional Overview
covering 475 km 2 . These 19 units were not selected randomly, but were based on the analysis of
opium cultivation in previous years. In total, 89 sampling segments were selected.
The sample size was calculated as a function of the costs associated with the helicopter flying
time and the precision of the estimate. 42 This was a compromise as a larger sample size leads to
greater accuracy in the estimate. However, financial resources limited the size of the sample.
When selecting the samples, the sampling frame was stratified in geographical clusters to
optimize the geographical distribution. To obtain a geographically well distributed sample while
being able to estimate the variance (uncertainty) in an unbiased manner, a stratified,
probabilistic sampling approach was adopted to select the 70 sampling units. These samples
covered all the risk area, apart from the hotspot area. The frame was divided into compact
geographical strata of approximately equal area by applying a clustering algorithm (“k‐means”) in
the statistical software R. 43 In each stratum, two sampling locations were selected by simple
random sampling, making it possible to calculate the variance in each strata.
Table 5: Number of sampled segments surveyed by helicopter in northern Lao PDR, 2014
Province
Grand total
BOKEO 4
HOUAPHAN 14
LOUANG NAMTHA 8
LOUANGPHRABANG 12
OUDOMXAI 10
PHONGSALI 10
XAIGNABOULY 2
XIANGKHOANG 10
Grand total 70
42
See the Methodology section in the Southeast Asia Opium Survey 2013, available at http://www.unodc.org/unodc/en/cropmonitoring/index.html.
43
http://www.r‐project.org/ and package http://cran.r‐project.org/web/packages/spcosa/index.html.
38