1. A method of controlling a system using control data stored in a data table within a controller memory unit indexed as a function of a measurable parameter of the system, the method comprising the steps of: establishing a hash table in said memory unit having hash index values; linking various values of said measurable parameter to said hash index values with a hash function; forming key elements for each of the linked values of said measurable parameter, each key element including a hash index value, an index value for said data table and a weight value; selecting for inclusion in the hash table only key elements for which said weight value exceeds a specified weight, and chaining the selected key elements to a respective hash index value in said hash table; obtaining a sample of said measurable parameter; determining a hash index value corresponding to said sample, probing the key elements chained to the determined hash index to locate a key element for said sample, and locating control data stored for said sample using a data table index value of the located key element; probing said data table to locate the control data for said sample if a key element for said sample is not located in said hash table; and controlling said system using the located control data.