Interest in a more accurate method than the the trapezium or trapezoid method for determining the area, F value,  under the lethal rate time plot has continued. Simpson's rules provide a more accurate way of calculating the area under the curve and an Excel spreadsheet has been programmed to use both rules to determine the F value of thermal processes and  is now  available for http://www.dairyscience.info/feedback/citefo.asp" rel="nofollow - download .  Fahrenheit and Celsius versions have been provided as has D, Z, and reference temperature data for major food spoilage organisms. Additionally a z-value converter to change temperature from Celsius to Fahrenheit and vice versa is available.