What is evidence? It is anything that demonstrates the soundness of a claim. Facts, statistics, expert testimony, and firsthand observations are four reliable forms of evidence. Other evidence might include examples, illustrations, details, and opinions. Depending on the purpose of your assignment, some kinds of evidence weigh more heavily than others. For example, readers might appreciate your memories of livestock care on the farm in an essay recalling your childhood summers. However, they would probably discount your memories in an argumentative paper about ethical agricultural methods unless you could show that your memories are representative or that you are an expert on the subject. Personal experience may strengthen an argument but generally is not sufficient as its sole support. If you are in doubt about the type of evidence an assignment requires, ask your instructor whether you should use sources or rely on personal experience and examples.

Facts. Facts are statements that can be verified objectively, by observation or by reading a reliable account. They are usually stated dispassionately: “If you pump the air out of a five-gallon varnish can, it will collapse.” Of course, we accept many of our facts based on the testimony of others. For example, we believe that the Great Wall of China exists, although we may never have seen it with our own eyes.

Sometimes people say facts are true statements, but truth and sound evidence may be confused. Consider the truth of these statements:

Some would claim that each statement is true, but when you think critically, you should avoid treating opinions, beliefs, judgments, or personal experience as true in the same sense that verifiable facts and events are true.

Statistics. Statistics are facts expressed in numbers. What portion of American children are poor? According to statistics from the U.S. Census Bureau, 13.2 million children (or 18.2 percent of all children) lived in poverty in 2008 compared with 16.4 million (or 22.5 percent) in 2011. Clear as such figures seem, they may raise complex questions. For example, how significant is the increase in the poverty rate over three years? Has the percentage fluctuated or steadily increased? What percentage of children were poor over longer terms such as fifteen years or twenty?

Most writers, without trying to be dishonest, interpret statistics to help their causes. The statement “Fifty percent of the populace have incomes above the poverty level” might substantiate the fine job done by the government of a developing nation. Putting the statement another way — “Fifty percent of the populace have incomes below the poverty level” — might use the same statistic to show the inadequacy of the government’s efforts.

Even though a writer is free to interpret a statistic, statistics should not be used to mislead. On the wrapper of a peanut candy bar, we read that a one-ounce serving contains only 150 calories. The claim is true, but the bar weighs 1.6 ounces. Gobble it all — more likely than eating 62 percent of it — and you’ll ingest 240 calories, a heftier snack than the innocent statistic on the wrapper suggests. Because abuses make some readers automatically distrustful, use figures fairly when you write, and make sure they are accurate. If you doubt a statistic, compare it with figures reported by several other sources. Distrust a statistical report that differs from every other report unless it is backed by further evidence.

Expert Testimony. By “experts,” we mean people with knowledge gained from study and experience in a particular field. The test of an expert is whether his or her expertise stands up to the scrutiny of others who are knowledgeable in that field. The views of Peyton Manning on how to play offense in football carry authority. So do the views of economist and former Federal Reserve chairman Alan Greenspan on what causes inflation. However, Manning’s take on the economy or Greenspan’s thoughts on football might not be authoritative. Also consider whether the expert has any bias or special interest that would affect reliability. Statistics on cases of lung cancer attributed to smoking might be better taken from government sources than from the tobacco industry.

Firsthand Observation. Firsthand observation is persuasive. It can add concrete reality to abstract or complex points. You might support the claim “The Meadowfield waste recycling plant fails to meet state guidelines” by recalling your own observations: “When I visited the plant last January, I was struck by the number of open waste canisters and by the lack of protective gear for the workers who handle these toxic materials daily.”

As readers, most of us tend to trust the writer who declares, “I was there. This is what I saw.” Sometimes that trust is misplaced, however, so always be wary of a writer’s claim to have seen something that no other evidence supports. Ask yourself, Is this writer biased? Might the writer have (intentionally or unintentionally) misinterpreted what he or she saw? Of course, your readers will scrutinize your firsthand observations, too; take care to reassure them that your observations are unbiased and accurate.