Blushii is right.
I'd like to further emphasise the concept of "validity". Validity can be referred to your actual methods - which means "is this the right method to use to measure this?".
Validity can also be referred to your conclusion - meaning "from what you have found (your results), have you made an approrpiate conclusion based on what you've got?" For example, if you observed 10 trees with blue leaves (results) and you claim that all trees have blue leaves (conclusion), this is an invalid conclusion. Similarly, if you have tested 100 cows with disease ABC and 100 healthy cows, and you've found that all 100 cows with disease ABC have the E. coli bacterium and none in the healthy cows, and you conlcuded that E. coli is associated with disease ABC, this is a valid conclusion.
I hope this helps!