Patent practitioners understand the basic requirements for filing a continuation-in-part (CIP) application, such as having at least one common inventor with the parent application and co-pendency with the parent application. The MPEP as well as other sources on the Internet provide this basic information without explaining any benefits of a CIP, other than claiming priority to the parent application in the event you want to include a claim in the CIP that is supported solely by the parent application (similar to filing a continuation or divisional application).
However, the main benefit of filing a CIP (in my opinion) is to achieve consistency in the prosecution of both the parent and CIP applications. In my experience a CIP application is always assigned to the same examiner as the parent application (which makes sense from the USPTO viewpoint). That is when the USPTO processes a new CIP application, the unofficial procedure is to always assign the CIP application to the same examiner as the parent application. Why is this important? Well, if a follow-on application were assigned to a different examiner, you run the risk of inconsistent examination meaning different prior art applied to similar claims as well as different rejections. You don't want the examiner of the parent application finding claim elements patentable with a different examiner of a follow-on application finding the same claim elements unpatentable. In effect you end up with twice the effort (and cost) arguing patentability with two examiners instead of one, as well as twice the litigation effort and cost assuming you are eventually successful with both examiners. In addition if you fail in the prosecution of either the parent or a follow-on application, it obviously creates a cloud over any related issued patents.
I have also found that when successful arguments for patentability are achieved with the examiner of the parent application, that same examiner is much more amenable to allowing claims in a CIP application. Conversely when a follow-on application is filed without claiming priority to the parent (i.e., not as a CIP), you have to start the entire prosecution over if a different examiner is assigned to the new application.
For these reasons, I always recommend filing a follow-on application as a CIP even though it will limit the life of the CIP to that of the parent application.
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