Stratified Random Sampling STRS Example

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Stratified Random Sampling (STRS) Example
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Stratification in Fisheries Estimation

• A survey of fish was taken in a Louisiana
  lake. Fish were caught information on species
  and fish length was recorded. You have been
  asked to determine the average length for a fish
  within the lake, along with the standard error of
  that mean.

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Information

• Known
• Fish species
• Fish length
• Calculate
• Mean fish length
• Standard Error of Mean

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Using SRS

• Mean
• Variance
• Standard Error

44.43
206.74
1.53
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• However, we have information on species. It
  seems logical that length may vary between
  species and that perhaps we could use stratified
  sampling (STRS)to reduce our variation.
• Note In order to use STRS, we must estimate the
  total fish population within each stratum, which
  was the following Black Crappie 1500, Bowfin
  750, Largemouth Bass 525.

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Using STRS
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Using STRS continued
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Using STRS continued

• Even though the mean changed substantially
  (lowered by 17, which we would expect since we
  gave more weight to the smaller fish (Black
  Crappie)), we made the most significant gains
  with the standard error (lowered by 59)