Trying to save a value across polls

I am using SMARTS IP to poll network devices and have sucessfully modified the example in the tutorial to poll a variety of mibs and generate events and do calculations and create events based on simple thresholds.

However I need to save a value across polls to compare; i.e. an interface might be erroring at 1.5%, and I don't want to generate an event based on a simple threshhold, but only if the error goes up by .1% on the next poll So I need the error rate from the previous poll saved on a per-network-element basis. How can I do this in a .mdl file?

Thanks for your help.

Andy