Individuals / units
Most data sets consist of one or more values that are recorded from each of a set of individuals (or plants, plots of land, repetitions of an experiment or other 'units'). There are two different ways in which data can be collected from these units.
Types of data collection
In an observational study, we passively record (observe) values from each unit. Usually these units are sampled from some population.
In an experiment, the researcher actively changes some characteristics of the units before the data are collected. The values of some variables are therefore under the control of the experimenter. In other words, the experimenter is able to choose each individual's values for some variables.