Find and replace missing values with other attribute values

Sometimes, it is helpful to replace a numerical attribute containing a missing value with a value from another attribute in the example row rather than using a constant such as an average or maximum value. This process generates some missing values, detects them and replaces with a calculation based on the value of another attribute. (Purists will be alarmed at the mathematical impossibility of taking the square root of a negative number).

The process detects missing values by subtracting an attribute value from itself. If the value is missing, the result of this will be non zero whereas if the value is not missing, the result will be zero. This can be used to generate a new attribute.