Know What You are Looking For
Before we talk about the specifics of different patent searches, we need to define a very important patent searching concept: the "disclosure". The disclosure refers to the document which explains the invention or concept on which you are going to search. Depending on the search type, and your role in the patent search process, the disclosure may take different forms.
If you are an inventor conducting your own search, you might not have a disclosure at all. You might just have an idea in your mind as to what you are looking for. If you are a professional patent searcher, you will likely have a disclosure from your client or employer. If you are an inventor and have not written a disclosure, we suggest you do, even if just to get your own thoughts in order - it will help you search more effectively.
The disclosure for Novelty searches and Infringement searches describes the proposed inventions or products. The disclosure may be anything from informal document with rudimentary sketches of the invention to a complete patent application with professional drawings, or even an existing patent.
Based on the disclosure, you should be able to answer this question: What are the novel points of this invention? The reason for this is that you are going to be searching for prior art relevant to the aspects of the invention that are novel - which is not necessarily the entire invention.
For instance, given a disclosure about a bicycle with non-slip pedals, the points of novelty might be the particular shape of the pedals, or the materials that the pedals are made of, or some type of locking mechanism that keeps your feet from coming off. Of course, the disclosure may talk about bicycles in general, and the diagrams may include pictures of entire bicycles, not just pedals. That does not mean that any patent about bicycles is relevant. You must understand which features are common knowledge and simply present to put the invention in context versus which features are potentially patentable.
In a State-of-the-Art Search, the objective is often to find out what other people in the field are doing. A State-of-the-Art search may not be based on a specific invention at all, but rather the disclosure may simply be something like "Find all patents that discuss the use of magnesium alloys in bicycle frames".
The disclosure for a Validity search will normally be an existing patent. You will be given a patent number, and you will need to look the patent up, read it, and thoroughly understand the patent - particularly the Claims.
While disclosures may take different forms depending on the exact situation, it is always imperative that you know what you are looking for. It will save time, help make your search results concise, and help ensure that you do not miss anything relevant.

