Access 2003: assign a value according to a range of numbers

I am not an IT professional but have build databases for use by my department. My table contains a field called NAT Score which is filled with a number between 1 and 25. I would like reports to be able to sort and group records according to whether the NAT Score is high (>18), medium (>11 and <19) or low (<12), with the words 'high', 'medium' or 'low' appearing instead of the NAT Score number. I wondered if it was possible to populate another field from the NAT Score field or whether it would be better to create the field in the report. In any case, I cannot work out how to do either. Any suggestions would be welcomed. Thanks in advance.