A test may be reliable yet not valid, The results can end up being reliable, in other words certain to have yielded properly based on input. But the results may not be trustworthy.
In my view reliable test is always valid.
A test may be reliable but not valid. A test may not be valid but not reliable. For example, if I use a yard stick that is mislabeled to measure the distance from tee to hole in golf on different length holes, the results will be neither reliable nor valid. If you use the same stick to measure football fields that are the same length the result will reliable (repeatable, consistent) but not valid (wrong numbers of yards). There is no test that is unreliable (repeatable, consistent) and valid (measures what we are looking for).
Reliable indicates that each time the experiment is conducted, the same results are obtained (accuracy). Valid indicates the experiment (or test) has controlled variables and used an appropriate method/model.
A reliable measure is consistent and yields consistent results, so it may not be measuring the intended construct accurately (lack validity). On the other hand, a valid measure accurately assesses the intended construct, but it must be consistent and produce stable results (reliable) to ensure that the measurements are dependable and trustworthy.
A bathroom scale that consistently shows your weight as 10 pounds less than your actual weight, but always produces the same result when you step on it multiple times, can be considered reliable (consistent) but not valid (accurate).
A valid test is not always a reliable test. Validity refers to whether a test measures what it claims to measure, while reliability refers to the consistency of test results over time. For example, a test designed to measure mathematical ability may be valid if it accurately assesses math skills, but if the test yields vastly different scores when taken multiple times by the same individual, it lacks reliability. Thus, a test can be valid in content but still unreliable in execution.
Is it possible for an operational definition to be valid but not reliable
I think that with reliability we mean that the plans tha you propose are based on some spesific and realistic elements. With validity I think that we mean that these elements are true and modern.
You can tell that a test is reliable when many can take it and it holds up. A reliable test can't be biased, and it must measure what it says it's going to measure. A reliable test stands up when compared to other tests.
So they can be sure that the data they are releasing to the rest of the scientific community and world is actually correct.
There is no such valid test, so I would definitely question it.
No