Doesn enhancing the sampling intensity for an area of special interest bias the overall estimates?
No. Sampling units inside an area of special interest usually have a higher chance of being selected than sampling units outside the special interest area. Within each stratum, however, the chance of selecting any location is equal; therefore, a separate (unbiased) estimate can be computed for each stratum, as well as for the entire resource. With stratified random sampling, estimates are generated first for individual strata, then the stratum-specific estimates are combined into an overall estimate for the whole target population. Stratum-specific estimates are combined by weighting each one by the fraction of all sampling units that are within the stratum. For the simple two-stratum example given above, the weights would be 200/1000 for stratum 1 and 800/1000 for stratum 2. So, if the stratum-specific estimates are 0.5 for stratum 1 and 0.25 for stratum 2, the overall estimate is 0.30 [(O.5 x 2/10) + (0.25 x 8/10)]. This approach ensures that the overall estimate is corrected for the
