Percent yield offers how much of the expected quantity is created. Percent yield is actual yield divided by theoretical yield. Theoretical yield comes from stoichiometry whereas actual yield comes from lab results. Percent yield can be lower than 100% when some of the initial quantity is not fully reacted, or if say some of the sodium chloride in a water solution is boiled off with the water. Percent yield can be greater than 100% if there are impurities in the yield. That is, an impurity that remains in the final compound can add to the weight and give you over 100% yield.