Which condition is necessary for an experiment to yield useful data?

Answer;

-Similar results should be obtained when the experiment is repeated.

Explanation;

-A valid experiment is repeatable; when you repeat your experiment, it should yield the same results. It is also reproducible. Someone else should be able to perform your experiment and get the same results.

-If other scientists get different results, then you have overlooked one or more important parts of your experimental design, like a variable you didn't take into account that influences the outcome.