Full Answer Section

• History: This occurs when an event that happens outside of the study confounds the results. For example, if a new police chief is appointed in the middle of the study, and this new chief implements a new policing strategy that reduces crime, then the results of the study may be attributed to the police intervention, when in fact they may be due to the new policing strategy.
• Maturation: This occurs when the participants in the study change over time, regardless of the intervention. For example, if the crime rate in a city is declining, then the crime rate in hot spots may decline even if there is no police intervention.
• Attrition: This occurs when participants drop out of the study. If participants who drop out of the study are different from those who remain in the study, then this can bias the results.
• Diffusion of benefits: This occurs when the treatment group benefits from the intervention, but the control group also benefits. For example, if the police increase their presence in a hot spot, then this may deter crime in the surrounding area, even in the control group.

By being aware of these validity threats, researchers can take steps to minimize their impact on the results of their studies. For example, researchers can use random assignment to assign participants to the treatment and control groups, and they can monitor the study to ensure that participants do not drop out.

In addition to the validity threats listed above, researchers should also be aware of other threats to the internal and external validity of their studies. For example, researchers should be careful to measure the outcome of interest in a reliable and valid way, and they should ensure that the results of their studies are generalizable to other settings.