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August, 2002 Are "Thief Samples" legal? Yes! But thief samples are probably not what you think they are. A thief sampler can take a core sample of solid material either vertically (from a drum) or horizontally (from a bag). Not all thiefs look the same, but here is a rough drawing of one type.
How it works: The point is used to push through the material. The "windows" are closed when pushing through the material, then opened to allow material to flow in at the different levels. After closing the windows, the thief is pulled out. You are probably familiar with trying to sample from a drum of liquid or solid material. With solids, scooping off the top is convenient (a "grab" sample). With liquids, using a spigot at the bottom is common. However, this type of sampling is not correct because not every part of the material in the drum has an equal chance of being in the sample. In other words, these types of samples are not random. These samples will not be representative, especially if there is variation from top to bottom due to gravitational effects on the shape, size, or density of the material. Advantages of a thief sampler:
Limitations of a thief sampler:
Moral: While a thief sampler has drawbacks, it is an improvement over grab sampling. We cannot always get a truly random sample, but we can reduce the bias in our samples by improving our equipment and procedures. Sampling Challenge: Where did the thief sampler get its name? I don't know!! If you know or have any idea, please let me know. I'll share your knowledge with others in my next E-News. More information about
sampling equipment and procedures for solids, liquids, and gases is
available in the Short
Course September 16-17, 2002, at Texas Tech University and
in the short introductory book A
Primer for Sampling Solids, Liquids, and Gases. Send This Information To a Friend "Your decisions are
only as good as your samples." Newsletters | About
the Owner | Bulk
Sampling Short Course
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