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'). These individuals will vary in many ways other than the variables that are recorded.

There are two different ways in which data can be collected from these units.

Observational studies

Data are collected in an observational study if we passively record (observe) values from each unit.

Most observational studies are conducted by sampling units from some population.

Share prices

The scatterplot below describes the share prices for 60 randomly selected companies in two successive years.

Click Take sample a few times to see the share prices of other another 60 companies in the two years. Observe that both variables vary from sample to sample since the data are observational.

Experiments

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.

Type of experiment Possible controlled variables
Business Shelf layout in supermarket
Price of apples
Number of TV adverts
Psychology Time allowed to memorise text
Type of stimulus in reaction-time test
Industrial Temperature of chemical reaction
Quality of raw materials for a process

Lathe experiment

A quality control engineer is concerned about complaints from customers about the surface finish of a metal part that is produced on a lathe. She conducts an experiment to determine whether alterations to the speed (in RPM) of the lathe may help. Four parts are produced at each of 8 lathe speeds, and their surface finish is measured.

This is experimental data since the engineer can control the lathe speed (the explanatory variable).

Click Take sample a few times to repeat the experiment and observe that the distribution of lathe speed remains the same — only the response (surface finish) changes in repetitions of the experiment.