In measurement, no instument can procure a precise measurement, so you will have a limit of reading for each variable that you are measuring- this referred to the uncertainty (half the limit of reading). When doing calculating the value you are finding, you also calculate the uncertainty for the value (through conversion to percentage) which will give you a range of uncertainity. Providing you have been concise when conduction the experiment, the commonly accepted value should be within this range. In addition to this is whether the substances used are pure or mixtures. If the substance is not pure (eg brick, alloy etc) then more discrepency can be allowed though the average ratio of the pure substances that make up one peice of brick to another should be similar, given the sample is from the same source as the two compared.