Why sampling

Table of Contents Welcome Introduction: Epidemiology in crises Ethical issues in data collection Need for epidemiologic competence Surveys - Introduction Surveys - Description of sampling methods Sampling unit What is sampling?

Why do sampling? A survey without sampling Population of Papua New Guinea census : , children less than 5 years of age 1,, women years of age If one survey team could collect data on 13 women and 13 children per day, 6 teams would take 16, days or The choice of sample items depends exclusively on the judgement of the investigator.

The investigator exercises his judgement in the choice and includes those items in the sample which he considers most typical for his investigation. For example, from a rice field one investigator may select only the healthy plants for artificial inoculation of a pathogen. Top Menu BiologyDiscussion. Biometry: Application and Limitation Statistics.

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These cookies will be stored in your browser only with your consent. You also have the option to opt-out of these cookies. Random sampling is much faster than surveying everyone in a population, and obtaining a non-random sample is almost always faster than random sampling. Thus, sampling saves researchers lots of time.

The number of people a researcher contacts is directly related to the cost of a study. Sampling saves money by allowing researchers to gather the same answers from a sample that they would receive from the population. Non-random sampling is significantly cheaper than random sampling, because it lowers the cost associated with finding people and collecting data from them. Because all research is conducted on a budget, saving money is important. Sometimes, the goal of research is to collect a little bit of data from a lot of people e.

At other times, the goal is to collect a lot of information from just a few people e. Either way, sampling allows researchers to ask participants more questions and to gather richer data than does contacting everyone in a population. Efficient sampling has a number of benefits for researchers.

But just as important as knowing how to sample is knowing where to sample. Some research participants are better suited for the purposes of a project than others. Finding participants that are fit for the purpose of a project is crucial, because it allows researchers to gather high-quality data.

For example, consider an online research project. A team of researchers who decides to conduct a study online has several different sources of participants to choose from. Some sources provide a random sample, and many more provide a non-random sample. When selecting a non-random sample, researchers have several options to consider.

Some studies are especially well-suited to an online panel that offers access to millions of different participants worldwide. Other studies, meanwhile, are better suited to a crowdsourced site that generally has fewer participants overall but more flexibility for fostering participant engagement. In essence, we want to have a little group that is like the big group. With that in mind, one of the features we look for in a sample is the degree of representativeness - how well does the sample represent the larger population from which it was drawn?

How closely do the features of the sample resemble those of the larger population? There are, of course, good and bad samples, and different sampling methods have different strengths and weaknesses. Before turning to specific methods, a few specialized terms used in sampling should be defined. Samples are always drawn from a population, but we have not defined the term "population. The population to be sampled the sampled population should coincide with the population about which information is wanted the target population.

Sometimes, for reasons of practicality or convenience, the sampled population is more restricted than the target population. In such cases, precautions must be taken to secure that the conclusions only refer to the sampled population.

Before selecting the sample, the population must be divided into parts that are called sampling units or units.