Validity of the experiment: Our experimental procedure tested the hypothesis and found the concentration of sulfate ions in the fertilizer. Thus it appears to be valid. However this meant that; we assume each step of the procedure was perfect e.g. the entire sulfate was precipitated and accounted for, there were no material losses, only the sulfate precipitated with the barium and there were no other interfering substances etc. but this is not possible. So the experimental procedure was invalid because there are high chances of errors. I am confused how the above is about the validity of the experiment? should it not come under reliability?