There are many tools associated with paranormal investigation. EMF meters, video cameras, audio records the list grows every year. For many interested in the field it can seem daunting having to spend possibly hundreds of dollars on technology they don’t even understand why they are buying meanwhile far too many investigators both new and experienced ignore the most important tool they have: their own rational mind.
Critical thinking is an indispensible part of being an investigator (and really life in general). For an investigator to jump to quickly to the conclusion that a house is haunted at best will make them look extremely foolish at worst may encourage someone with delusions in their mistaken beliefs. Granted being to skeptical, despite what some doubters would say, is no better than being too quick to believe but it’s important to evaluate any phenomenon carefully to be sure it cannot be explained naturally. The point is striking a balance between the two extremes being at least willing to accept that such phenomenon can happen while always on the lookout for another explanation.
There are a great many books on critical thinking and the subject is too complex for a single article but there are some basic principles its valuable to keep in mind. The first is the concept of deductive reasoning as opposed to inductive. Inductive reasoning is one of the most common mistakes investigators of paranormal phenomenon make. It’s when an investigator begins with a conclusion (a person is/isn’t psychic, a house is/isn’t haunted) and then seeks to prove that theory. The problem here is that the natural human tendency is to ignore evidence that doesn’t support an already decided upon conclusion. A believer might for example dismiss evidence that symptoms of “possession” may in fact indicate mental illness while a skeptic may stubbornly insist that the witness must be lying because ghosts don’t exist. Instead the goal should always be deductive reasoning. In deduction an investigator gathers evidence and then draws a conclusion based on what is found. So in a reportedly haunted house they wait, document what occurs and then draw conclusions based on the documentation. It’s harder for bias to creep in here. This doesn’t mean assuming the house is not haunted (again this is biased and falls into inductive reasoning) it means waiting for the evidence before drawing a conclusion at all. Even taking it as a given that ghosts and the like do exist that does not mean every strange experience a person has will always be caused by ghosts.
The next most important principle is what’s called parsimony or Ockham’s Razor. To quote Sir Isaac Newton this means“"We are to admit no more causes of natural things than such as are both true and sufficient to explain their appearances. Therefore, to the same natural effects we must, so far as possible, assign the same causes." In other words when explaining an unusual occurrence it’s more likely to be the result of an already understood phenomenon rather than something outside of current understanding. This is not a hard and fast rule, certainly many great leaps in scientific understanding would never have survived its application, but the founding principle is extremely useful. A perfect example of the value of Ockham’s Razor can be applied to one of the more ridiculous “arguments” in the paranormal community: orbs. Orbs are circles of light that appear in photographs, most often digital photos to be precise. Some believers in paranormal phenomenon suppose that these are some kind of spirit being while others believe them to be particles in the area like dust, moisture, pollen and so on. Believers will retort that some orb photographs are clearly dust and that others are real and suppose various ways of telling which is which. Setting aside the arguments based on the technical aspects of photography parsimony to determine the more likely explanation. For orbs to be spirits one must assume that: 1) Spirits exist, 2) Spirits can be photographed, 3) that when spirits are photographed they at least some of the time manifest in the form of circles of light. For it to be dust no assumptions are required since its well established that dust exists and that when photographed it looks like these supposed orbs. This alone forms one of the most basic reasons skeptics generally laugh at the idea that orbs are anything paranormal. Even for a rationally minded believer orbs are poor evidence since they occur naturally. Again though parsimony is a principle not a hard and fast rule, It is not logical to try to shoe horn something into a neat and easy explanation just because that explanation fits into one’s world view (for an excellent example of this compare Larry Kusche’s explanation in “The Bermuda Triangle Mystery: Solved” to some of the more elaborate ones that have been offered).
None of this is to say a person should rule out the possibility of the paranormal just because it doesn’t fit into their world view. Certainly one of the common arguments against paranormal phenomenon (and for that matter God) that the lack of evidence is itself proof it does not exist is not itself a illogical (logic draws conclusions based on facts and premises not on the absence of them) just that its vital to not rush to judge what seems unusual as paranormal. Instead rely on the advice of the fictional detective Sherlock Holmes “When you have eliminated the impossible, whatever remains, however improbable, must be the truth.”
Ockham's razor: http://math.ucr.edu/home/baez/physics/General/occam.html
Logical fallacies: https://owl.english.purdue.edu/owl/resource/659/03/
Kusche's explanation of the Bermuda Triangle: http://www.unmuseum.org/triangle.htm