Like biologists, or physicists, many criminologists also follow the scientific method. A criminologist starts by observing the world and recognizing a problem in need of explanation. One way to construct theory is to collect empirical evidence (collecting evidence by observing the world). For example, two of the early founders of the Chicago school, Robert Park and Ernest Burgess, spent time in Chicago in the 1920s and made observations about the location and characteristics of Chicago neighborhoods. They observed that the neighborhoods away from the city center had higher socio-economic conditions and relative absence of crime; in contrast, the inner-city neighborhoods had lower socio-economic conditions and higher levels of crime.

When criminologists find an interesting pattern, as Park and Burgess did, they try to make a hypothesis to explain why this pattern exists. Park and Burgess proposed that there is a particular way in which large cities are organized. They claimed that poor neighborhoods tend to be in the center because that is where all the industrial work sites, like factories, are usually located, and the residents need to remain close to their work. Wealthier neighborhoods, on the other hand, tend to be located on the outskirts, away from dirty industrial areas, in neighborhoods that are usually cleaner and safer.

The hypothesis then needs to be tested, often by collecting quantitative data (information that can be measured with numbers). The data also needs to be interpreted to see whether it supports or disproves the hypothesis. If the interpretation of the data does not support the hypothesis, the hypothesis must be changed or abandoned. In any case, theories should be testable. Otherwise, the theory may be considered as one person’s view of the world, or an expression of her/his prejudices. In addition, if a theory is testable, other researchers can then work to improve it.