An analogical argument is the use of a comparison between two things which have some similarity and from this basis inferring that they share some other property. These kind of arguments are common in both philosophy and in everyday life. The general form of an analogical argument is:
A has properties p, q, r
B has properties p, q, r
A has property s
Therefore B has property s
The strength or weakness of an analogical argument depends upon a number of considerations:
Similarity - Verify that the properties proposed as being shared among the comparison group do indeed exist. As analogical arguments are rarely actually presented in the form above, it may even be necessary to first list just how it is assumed the comparison groups are similar. Here is a simple example. "John is like Mike. Mike is smart. Therefore John must be smart". In this example none of the assumed similarities between John and Mike have been presented. Before the argument can carry any weight these similarities must be listed and verified.
Relevance - The more relevant the comparison properties are to the target properties, the stronger the argument. Here is an example of an analogical argument which lacks relevance. "Book A and Book B both have a hardbound cover, pages, words on the pages and numbers at the bottom of the pages. Book A is a boring story. Therefore we can assume that Book B has a boring story." Though I have given a number of similar properties between Book A and Book B, none of these properties are relevant and thus do nothing to increase the probability that Book B is boring.
Number - The more properties two things share in common with each other, the stronger the argument. This is based on the notion that the more two things are alike, the more likely they also share the property stated in the conclusion. As stated above, relevance plays a key role in determining how much weight these similarities are given.
Disanalogy - Relevant disanalogies or dissimilarities must also be considered when determining the strength or weakness of an analogy. For example if I say, "I have known three people who have had surgery at this hospital with the same surgeon and they have all turned out successfully. Therefore Jane's surgery will also be a success." But what if the three success stories all had minor surgery and Jane is scheduled for a high risk procedure? This of course would be a very relevant disanalogy.
This brings us to the somewhat misleading fallacy known as the False Analogy. It is misleading in the sense that the wording can cause a person to think in terms of either good or bad. Though the term False Analogy is the most popular name for this fallacy, Weak Analogy is probably a better name. Analogies are best analysed in terms of strength or weakness. With this in mind, a False Analogy is essentially a very weak analogical argument.