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Statistical Principles and Sampling Design

Statistical Principles and Sampling Design. Y. X. 3. Sampling Design. A procedure for selecting events from a population. Pilot sample (or pretest) Simple random sample random number generators are handy for x/y selection. x. x. x. x. x. x. x. x. x. x. x. x. x. x. x. x. x.

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Statistical Principles and Sampling Design

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  1. Statistical Principles and Sampling Design

  2. Y X 3. Sampling Design • A procedure for selecting events from a population • Pilot sample (or pretest) • Simple random sample • random number generators are handy for x/y selection

  3. x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x 3. Sampling Design • Systematic random sample • select k-th individual from gridded population • lay out a line = transect, sample individual nearest the pre-selected point

  4. 3. Sampling Design • Stratified random sample • population is layered into strata and then we conduct random or systematic sampling within each cell

  5. 8 4 5 1 8 1 9 5 2 3. Sampling Design • Stratified, systematic, unaligned = point sampling • Hybrid technique, favored among geographers

  6. 8 4 5 1 8 1 9 5 2 3. Sampling Design • Stratified, systematic, unaligned = point sampling • Hybrid technique, favored among geographers

  7. x x x x x x x x x x x x x x x x x x x 3. Sampling Design • Transect = line sampling, but lay-out must have a random component! (How can this be accomplished?) • Many variations: • Sample all individuals along the transect (row 1) • Sample quadrats along the transect (row 2) • Sample all individuals within a belt (row 3)

  8. 3. Sampling Design • Targeted sampling = non-random sampling • Is this a legitimate technique? • It is often necessary because of: • Time constraints • Budget constraints • Lack of field labor • Physical limitations of field labor • Topographic limitations • Advantages? • Maximize information with minimum resources • Target areas where samples are known to exist • Less time needed and less money wasted

  9. 3. Sampling Design • Targeted sampling = non-random sampling • Used in practically all types of dendro research: fire history, climate reconstruction, insect outbreak studies, …

  10. X X X X X X X X 3. Sampling Design • Specifically sample only trees that have best record of fire scars. (dots = trees, circles = trees collected with fire scars, X’s = fire scars, but not sampled = poor record.) • What issues must we consider? Topography, slope, aspect, hydrology, tree density: all affect susceptibility to scarring by fire. Shallow slope area Valley bottom Steep slope area

  11. 3. Sampling Design • Complete inventory is possible • Sample all trees that have fire scars, regardless of number of scars or quality of preservation, but … • Not very efficient (time, money, labor) • Benefits are considerable, though. Shallow slope area Valley bottom Steep slope area

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