What technique is used when individuals are selected according to a constant determinant, such as every fourth subject?

The technique described is systematic sampling, where individuals are selected based on a constant interval or predetermined rule, such as every fourth subject. This method allows for a straightforward way to gather a sample from a larger population without the necessity of random selection for every individual. By using a fixed interval, researchers can streamline the sampling process, making it efficient and ensuring that the subjects selected are representative over a span of the population.

Systematic sampling can improve the operational efficiency of the selection process compared to other sampling methods. For instance, unlike simple random sampling where each subject is selected independently and purely by chance, systematic sampling provides a methodical way to ensure coverage across the population while reducing the complexity of randomization. The use of clusters and stratification, as seen in the other options, involves different approaches to grouping or categorizing individuals, rather than employing a consistent interval for selection.