What Is A Sampling Bias?

Sampling in ecology is the act of quantifying or measuring populations of organisms in a given area. Sampling is often used to measure the biological diversity of plant and animal species and can help us estimate their density, distribution, or migratory patterns. It is important to note that not all species are equally detectable. Generally, accounting for a species is easier if the species is larger in size or has greater abundance of individual members.

All ecological sampling surveys begin with an objective. Objectives are specific questions that the sampling survey is designed to answer. In other words, objectives provide the reason for quantifying a population in the first place.

Objectives also inform the structure of the sampling survey. The objectives dictate what will be measured, what strategies may be optimal in taking those measurements, or how much ground should be covered in the study. Without objectives, the survey data could become unfocused or useless altogether.

Ecological Sampling Bias

The sampling frame is the list of the organisms (plants, animals, bacteria, or insects) of a population from which a survey is taken. Sampling frames vary between studies because the studies themselves are usually designed to answer different questions about a target population.

Target populations are the ecological resource, the species or organism, of interest. When studying a target population, ecologists have to consider the bias involved in their study. A sample is biased if all individuals from the sampling frame were not equally likely to have been detected.

A survey team commits a measurement bias if they only account for a fraction of the sampling frame. This sort of measuring error would give ecologists values that do not completely cover the scope of the sampling surveys objective, and therefore could leave out crucial data points.

Biases are the result of taking a sample that fails to include all groups of the target population. Every sampling method and every sampling survey contains some sort of bias. Meaning that not all target populations and organisms will ever be measured with equal accuracy. Sampling biases might occur because some species are easier to detect than others, or because there is a particular set of limitations to consider when sampling a given species. Understanding biases is crucial for assessing the trustworthiness of a sample.

To rectify sampling biases, ecologists may double sample. Double sampling involves randomly choosing certain species from the total survey and measuring them a second time. Double sampling tests inconsistencies and potential defects in survey analysis.

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