Analytical Chem istry - DePauw University

Chapter 7 Collecting and Preparing Samples293St r a t i f i e d Sa m p l i n gAnother combination of the three primary approaches to sampling is judgmental–random,or stratified sampling. Many target populations consistof distinct units, or strata. For example, suppose we are studying particulatePb in urban air. Because particulates come in a range of sizes—some visibleand some microscopic—and from many sources—road dust, diesel soot,and fly ash to name a few—we can subdivide the target population by sizeor source. If we choose a random sampling plan, then we collect sampleswithout considering the different strata. For a stratified sampling, we dividethe target population into strata and collect random samples from withineach stratum. After analyzing the samples from each stratum, we pool theirrespective means to give an overall mean for the target population. Theadvantage of stratified sampling is that individual strata usually are morehomogeneous than the target population. The overall sampling variancefor stratified sampling is always at least as good, and often better than thatobtained by simple random sampling.Co n v e n i e n c e Sa m p l i n gOne additional method of sampling deserves brief mention. In conveniencesampling we select sample sites using criteria other than minimizingsampling error and sampling variance. In a survey of rural groundwaterquality, for example, we can choose to drill wells at randomly selected sitesor we can make use of existing wells, which is usually the preferred choice.In this case cost, expedience, and accessibility are more important thanensuring a random sample.7B.2 What Type of Sample to CollectAfter determining where to collect samples, the next step in designing asampling plan is to decide what type of sample to collect. There are threecommon methods for obtaining samples: grab sampling, composite sampling,and in situ sampling.The most common type of sample is a grab sample, in which we collecta portion of the target population at a specific time and/or location, providinga “snapshot” of the target population. If our target population is homogeneous,a series of random grab samples allows us to establish its properties.For a heterogeneous target population, systematic grab sampling allows usto characterize how its properties change over time and/or space.A composite sample is a set of grab samples that we combine into asingle sample before analysis. Because information is lost when we combineindividual samples, we normally analyze grab sample separately. Insome situations, however, there are advantages to working with a compositesample.One situation where composite sampling is appropriate is when ourinterest is in the target population’s average composition over time or space.