Successful persuasive writing features two key ingredients--sound argument along with valid, competent evidence. Evidence refers to the various facts the writer offers in support of the case he or she is trying to prove--or disprove. The optimal evidence presented is said to be "square on all four corners of the evidence" or, more simply, "square on all fours" if it meets all four requirements of validity presented here. What makes evaluation of evidence tricky is that the various criteria are not absolute black and white choices, but rather, tend to be shades of grey. So the process of evidentiary evaluation is one of weighing the importance of each criterion in the context of the presented case--just as a trial jury must do. Presented below are the four corners, or requirements, of evidentiary validity.
Relevancy - The First Requirement (or Corner) of Valid Evidence
Relevance is the measure of the tendency of the offered evidence to influence the outcome of the argument to a greater extent than if the evidence was not considered. For example, if the subject was a comparision of dog breeds for a thesis that a certain breed was the best for a certain purpose, information suggesting that cats make better pets would be completely irrelevant. Regardless of the other characteristics of this piece of evidence, cat evidence is not useful in making dog decisions. The rule is that evidence should be relevant to be valid.
Materiality - The Second Requirement (or Corner) of Valid Evidence
Materiality is the measure of the tendency of the offered evidence to help prove or disprove the argument. Material evidence will strengthen an argument while immaterial evidence does not. Continuing with the hypothetical dog essay, information concerning the amount of water different breeds consume daily is not material although the data could be relevant, credible, and reliable. This is because water consumption is directly related to the size of the animal, so it is logical that larger breeds would consume a greater amount of water. As the evidence is evaluated, evidence that is strongly related to other evidence does not assist in making the decision when compared to evidence assessing a variety of different qualities such as cost, trainablity, disposition, or size. The rule is that evidence should be material to be valid.
Credibility - The Third Requirement (or Corner) of Valid Evidence
Credibility is the measure of the tendency of the offered evidence to be believable in the context of the argument. It is on this corner that the veracity or truth of the evidence comes into play--evidence that is true is believable while evidence found to be questionable or false is unbelievable. Again, considering the hypothetical dog essay previously discussed, if a claim was raised that poodles can earn sixty thousand Dollars a year, such a statement is so unbelievable and obvioulsy false that it could not be considered in the evaluation. The rule is that evidence should be credible to be valid.
Reliability - The Fourth Requirement (or Corner) of Valid Evidence
Reliability is the measure of the tendency of the offered evidence to be repeatable. By repeatable, we mean that every time we observe the evidence what we find is exactly what was found every other time. For example, if we precisely measure how much milk is in a typical gallon container, repeat the measurement a number of times, and find that each container measured contains exactly the same quantity, we can reliably state that a typical gallon milk container contains exactly one gallon of milk. The rule is that evidence must reliable to be valid.
Strategies to Challenge Substance
Challenging the validity of a piece of evidence involves the application of different strategies presented here. The test for irrelevance is that if the evidence is removed from the argument, the outcome does not change. The test for immateriality is that argument is weakened without it. The test for incredibility is to show that the evidence presented is not true. Finally the test for unreliability is to demonstrate the evidence is not repeatable. Evidence that is found to be relevant, material, credible, and reliable is said to be valid; however, if the offered evidence fails one or more tests, it is said to be invalid. Conversely, the writer will ensure that all important evidence he or she presents is shown to be square on all fours to build a strong case.
The Characteristics of Competent Evidence
In addition to these various validity requirements, valid evidence must also be competent. Competent evidence has three main characteristics: it is authentic, complete, and best. Where the validity requirements focus on the substance of the evidence, the characteristics focus on the form of the evidence.
Authenticity or provenance--the evidence is what it says it is. Authentic evidence has an indisputable pedigree. For example, in a criminal trial, a chain of custody of evidence must be maintained to ensure that no tampering has happened. If the chain is broken, perhaps through faulty record keeping, the evidence may not be used at trial and the case lost. The rule is that evidence must be authentic to be competent.
Completeness--the evidence represents the whole of the relevant facts. The concern of completeness is that a writer might attempt to conceal unfavorable portions of the evidence that tend to weaken his or her argument. The rule is that evidence must be complete to be competent.
Best--the closer the evidence is to the original the better it is. Primary sources are the actual documents being presented; say for example, the original parchment of the Declaration of Independence. Secondary sources are the various publications that present information about primary sources. So, for example, a book about the Revolutionary War would be a secondary source. This is why hearsay (repeating what someone else said) evidence may not be an acceptable substitute for direct testimonial evidence. The rule is that evidence must be the best available to be competent.
Strategies to Challenge Form
To challenge the form of the evidence, begin by attacking its authenticity. Is it the real document, or a copy, or a forgery? Next, attack its completeness by showing that relevant and material facts have not been presented. Finally, challenge whether the evidence presented is the best available. Was the evidence obtained from a secondary source when a primary source was obtainable? Evidence that is found to be authentic, complete, and best is said to be competent; however, if the offered evidence fails one or more tests, it is said to be incompetent. Conversely, the writer will ensure that all important evidence he or she presents is shown to be competent to build a strong case.
Types of Evidence
All evidence falls into one of two categories--quantitative or qualitative. Quantitative evidence is concerned with the measurable properties of the something, qualitative evidence is concerned with various qualities of something. In this context, a quality is another word for a characteristic rather than an assessment of merit. So, in other words, by qualitative and quantitative, we mean numbers and words. Typically, qualitative evidence is either scientific or statistical in nature. Qualitative evidence, on the other hand, is typically is derived from document analysis; however, other methods used include anecdotes, observations, narratives, interviews, portraiture, or from focus groups. Regardless of the type of evidence used, there are a number of considerations to consider. First, is the type of evidence selected appropriate for what it is intended to prove? Second, has the correct method of data collection been selected? Third, was the data collection method used correctly? Fourth, were appropriate controls used to ensure that the collected data has not been influenced by external factors? Fifth, has an appropriate method of data analysis been selected? Sixth, has the data analysis method been implemented correctly? Seventh, does the analysis method include appropriate controls to ensure validity? Finally, are the results of the analysis significant? Whether the task is assessing evidence offered in aid of making a decision or proffering evidence to another in support of an argument, these understanding these concepts will prove useful.
Conclusion
Appropriate reasoning is only half of a sound argument--the use of valid and competent evidence is equally important. Valid evidence is relevant, material, credible, and reliable. Competent evidence will be authentic, complete, and best. A number of logical fallacies are directly concerned with problematic evidence as well. Understanding these measures of evidence will assist the writer in the evaluation of research materials as well as guide the presentation of these materials in her or his written product.
By Douglas E. Greathouse, MA.
