Descriptive investigations involve collecting data about a system, but not making

Descriptive investigations involve collecting data about a system, but not making tests or experiments.

Making tests belongs to experimental investigations.

Descriptive investigations aim to draw conclusions from observations of the fenomena, while experimental investigations design tests that let the researcher to compare results on different conditions with a control situation.

There is also the so called comparative investigations which collect data from under different conditions but do not include the control situtation.