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Current study participants recruit future participants from among their acquaintances. Use when researching hard-to-reach or hidden populations.

The population is divided into clusters, then a random selection of clusters is made. From those selected clusters, all or a random selection of units are surveyed. Use when the population is large and spread out geographically.

A more complex form of cluster sampling where several levels of units are embedded. Use for extremely large and geographically dispersed populations.

A method of sampling elements in a region whereby sample points are chosen by intersecting the region with a randomly placed line or transect. Use for ecological and environmental surveys.

Select every k-th individual from the population list. Use when you want a simple method and the list of the population is available.

Observations are made at pre-defined regular intervals of time. Use when longitudinal data on patterns and trends within a population are needed.

The population is divided into subgroups (strata) and random samples are taken from each stratum. Use when the population is heterogeneous and you want to ensure representation across groups.

The researcher uses their expertise to select units that are representative of the population. Use when detailed insight from a specific subgroup is needed.

Participants in the research volunteer themselves. Use when you require quick and low-cost samples, and are not aiming for generalizeable results.

Sampling methods are adapted as the initial results are evaluated. Use when there are clusters of a population with a high variance and hidden patterns.

Samples are taken from a group of people easy to contact or to reach. Use when time, money, or accessibility is a significant constraint and precision is not a primary concern.

Each member of the population has an equal chance of being selected. Use when you have a well-defined population and want unbiased results.

Researchers divide the population into groups and then arbitrarily choose participants from each group until quotas are met. Use when time is limited, and some representation of subgroups is needed.

A sample where participants are chosen to take part in a survey over a set period of time. Use to measure changes in the sampled population over that time.