Know the Main Kinds of Evidence

In the last chapter you learned to tell evidence from a feeling. The next thing worth knowing is that evidence does not all look alike. It arrives in a handful of familiar forms, and once you can name them, you will spot evidence faster and know where to go looking for it when a claim needs support. This chapter introduces the four main kinds.

Start with the Four Main Kinds

Most of the evidence you will ever use falls into four broad kinds: data and measurements, direct observation, documented experience, and credible testimony and expert opinion. You already meet all four in ordinary life, often without labeling them. Naming them does two things: it helps you recognize evidence you might otherwise overlook, and it reminds you to look for more than one kind, since a claim backed by several kinds usually stands on firmer ground than one leaning on a single source.

Recognize Data and Measurements

Data is information recorded as numbers or counts, and a measurement is a single such record: a price, a temperature, a time, a tally. Its strength is that it is precise and hard to argue with, as long as it was recorded carefully.

Suppose you suspect your morning bus has become less reliable. If you jot down its arrival time every day for two weeks, you are collecting data. Say it shows the bus arrived more than ten minutes late on eight of ten mornings. That measurement does real work: it turns “the bus feels unreliable lately” from a grumble into a claim anyone could check, and it even tells you roughly how unreliable.

Recognize Direct Observation

Direct observation is evidence you gather firsthand, by watching or experiencing something yourself rather than being told about it. Its strength is immediacy: you are not relying on anyone else’s account.

Imagine you are learning to bake bread and your loaves keep coming out flat. You stand and watch one bake, and you see the dough sink the moment the oven door opens partway through. That firsthand sighting is direct observation. It supports the claim “opening the oven early is deflating my loaves” far better than a vague sense that something goes wrong near the end, because you saw the cause and the effect with your own eyes.

Recognize Documented Experience

Documented experience is a record, kept over time, of what actually happened: a logbook, a diary, a maintenance sheet, a set of notes. It is more than a single measurement because it captures a run of events, often with context about what was tried and what followed.

Picture a community garden that keeps a shared logbook of what was planted, when, and how each bed did. If someone claims “the raised beds produce more than the ground-level ones,” the logbook is documented experience that can settle it: several seasons of recorded harvests, not one lucky summer or one person’s memory. The record carries weight precisely because it was written down as things happened, not reconstructed afterward.

Recognize Credible Testimony and Expert Opinion

Testimony is evidence in the form of someone’s account or statement, and expert opinion is testimony from a person with relevant, demonstrated knowledge. Its strength is reach: you cannot personally measure or observe everything, so you often, and reasonably, rely on people who can.

Say you are deciding whether to take a common over-the-counter medicine alongside another one you already use. A pharmacist telling you the two are safe to combine is credible testimony: the pharmacist has relevant training, and the claim sits squarely inside their expertise. That carries far more weight than the same reassurance from a friend who simply “never had a problem” with it.

See the Four Kinds at a Glance

Here are the four side by side, each with the example from this chapter, so you can hold them in view together.

Let’s Recap!

• Most evidence falls into four main kinds: data and measurements, direct observation, documented experience, and credible testimony or expert opinion;

• These are the main kinds, not the only ones, and a single piece of evidence can belong to more than one;

• Data is precise and checkable, direct observation is firsthand, documented experience captures events over time, and testimony lets you draw on knowledge you do not have yourself;

• Testimony is only as strong as the speaker’s relevant, demonstrated knowledge;

• A claim supported by several kinds of evidence usually stands on firmer ground than one leaning on a single kind.

You can now name the kinds of evidence, but not all evidence is equally strong. The next chapter shows you how to weigh a piece of evidence by its strength and compare the kinds fairly.