Negative and positive results
A scientific study is performed in an attempt to support or negate the validity of a certain hypothesis. In the optimum case, the study has the character of a cross experiment – a certain result would support the studied hypothesis while the opposite result would throw it into doubt. In this case, the study provides only one of the answers, yes or no. However, very frequently, our studies have an asymmetric output. For example, one result supports our theory, but the opposite result does not mean anything at all. In this case, the study can yield the answer yes (no) or do not know. In the second case, this is called a negative result. You would probably like a specific example; here is one: If we do not manage to find a transition link in the palaeontological record, then this can mean that that link never existed, but it can also mean that a fossil has not been preserved, or that we have simply not found it yet. However, if we find the intermediate link, our hypothesis will be supported.