The systematic elimination of extraneous variables other than those you are interested in can be achieved in:

Experimental research achieves systematic elimination of extraneous variables through control and manipulation.

Randomisation and controlled conditions isolate the independent variable's true effect.

Qualitative research explores meanings in natural settings without such variable control.

Correlational research measures associations but does not manipulate or fully control variables.

Longitudinal research tracks the same subjects over time without eliminating extraneous factors.

This control gives experiments their strong internal validity.