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What're the meanings of these phrases?

An intervention is when someone enters a situation to change it, rather than having events proceed as they would have without interference. The "effectiveness of interventions" would refer to whether the change is successfully made to occur, or whether the change has a desirable outcome.

Independent variables are conditions that can be changed. For example, in a scientific experiment, where the temperature is changed to see how it affects a chemical's color, the temperature is an independent variable and the color is a dependent variable. If there were two independent variables, such as temperature and humidity, the "relative impact of these independent variables" would describe whether the temperature had a smaller or greater effect (or impact) on the color than did the humidity.

The third phrase means about the same as the second. For example if the strength of a material changed by a factor of 1000 over a one percent change in temperature, we might say there is a strong relationship between the variables of strength and temperature. If there were a small strength change over a wide temperature change, we would say there is a weak relationship.

I don't see how the last set of words makes a phrase.