Once you have completed gathering your data it's time to determine what it is that was experienced. In the case of Bigfoot or a creature sighting that often requires some knoledge of animal behavior. The first step is to rule out a misidentification of some animal which is native to the area.
The first consideration is any physical evidence. If you have tracks or impressions are they identified? You may want to check a reference manual for large cats such as mountain lions or even bobcats you can easily compare the tracks to those given in the guide. Your local game commission has information available that you might obtain at little or no cost regarding animals native to your area.
If you have obtained any hair samples they too may be compared with those in a reference manual or the game commission office. Sometimes though the sample may not be easily identified. it may have been damaged or exposed to the elements and difficult to use. For this you can contact someone in a biology department who may be able to do a more in depth analysis based on DNA or chemical composition of the sample. This is one case where expert contacts come in to play. They can often resolve the type of animal the sample came from. Most animal sighting turn out to be common animals native to your locale. This is why your first step is to rule these out; often this resolves your case and it need go no further.
Your other observations also are a part of the evidence. If you made casts or did compression tests of any imprints they can be used to determine the approximate size and weight of the creature. Even if you have no other physical evidence, tracks or impressions can be used to identify the common animals that are often responsible for these sightings.
A special consideration must be given for snow. If you have tracks found in snow you need to be aware that as snow melts the tracks enlarge. A track from a common house cat may seem to be from a much larger animal if melting is taking place. What doesn't change is the stride. The distance between the tracks will remain constant and is a good indication of the size of the animal which made them. factor this in to your final results.
As far as those other strange events that defy explanation, there really isn't any one thing that can be said to apply. Your analysis of the evidence will obviously depend on the nature of the evidence itself. The best way is try to duplicate the event while keeping records or photographs as the test is done. Does your hypothesis fit the event? Is it likely that what was reported can be duplicated under certain conditions? Can you duplicate those conditions? Finally if possible, do so. Do you obtain the expected results? If so you have probably resolved your case.
Once resolved, submit your logs and photos of the test to the client along with your final report. If you obtained any physical evidence as a part of your investigation, it too should be returned. The client should read your summary and state any comments. If he disagrees, then he should provide some reason why. If his reasoning raises any doubts you may want to consider looking into that, otherwise your work is done. Advise the client if anything new develops you will be available to look into that as it happens.