Reaction is not always 100 % complete, there are several factors that the Actual Yield is not equal with theoretical yield.  Theoretical yield is the expected amount of product/s if all limiting reagent has been consumed, while actual yield is the amount of product/s obtained based from actual reaction. For example in making cookies, you are expecting 100 pcs of cookies but only 95 pcs are made.  Therefore the actual yield is 95 pcs and the theoretical is 100 pcs.  There are many factors that can affect why actual yield is not equal with the theoretical yield.

To determine if how efficient is the chemical reaction, percent yield can be calculated.  Percent yield is the proportion of the actual yield and the theoretical yield, as shown in the formula below: