Audit Risk Meter

I've used TT for 20+ years?  I've always had a "low" indication on the audit risk, even when I was still working and/or taking itemized deductions.  For recent years where my income has had little change due to retirement income and taking the standard deduction filing jointly.  This year the audit risk meter indicated a "medium" risk.  That concerns me since not much has changed from previous years.  What factors does TT use in determining this "audit risk?"  I know the IRS selects returns at random to audit.  Since I've never been audited is this a/the factor?  I would like to know so "before" I submit my taxes if I can reduce my risk of being audited.