The FAR (Federal Acquisition Regulation) Cost Principles Guide was recently updated through FAC (Federal Acquisition Circular 2005-95 (January 2017) and is available for download here. The previous update was September 2014 (through FAC 2005-76). Here's how DCAA (Defense Contract Audit Agency) introduces the guide:
The FAR Cost Principles Guide – a chronology of revisions to FAR Part 31 since 1984 – is used to determine the cost principles in effect at time of contract award that may be necessary for audits of historical costs (e.g. Incurred Cost audits or Claims audits).The FAR Cost Principles Guide traces all of the changes to the Federal Acquisition Regulation cost principle through the inception of the FAR system in 1984. It is useful for determining the precise cost principle or the precise version of the cost principle in effect at the time a particular contract was awarded.
While a few cost principle have never changed since the initial promulgation (e.g. bad debt expenses and alcoholic beverages), most have undergone some form of revision. Sometimes these revisions are frequent and significant. . The Compensation cost principle (FAR 31.205-5) for example has been revised 40 times, most recently in September 2016. Some FAR cost principles have been eliminated (e.g. ADP equipment FAR 31.205-2 and Civil Defense Costs - FAR 31.205-5).
Remember, in most cases, the cost principles that apply are those that are in effect as of the date of contract award. Without a good cross-reference of the cost principles and their effective dates, it would be very easy to apply incorrect standards. Compensation is a good example. The lower compensation cap (discussed here) applies to contracts awarded after June 24, 2014. Many contractors (and subcontractors) are working on contracts awarded prior to that date and are not subject to the lower caps.