Threats to validity are factors that can affect the certainty of conclusions or inferences in research, especially of cause and effect. They can arise from the selection of groups, the presentation of treatments, the measurement of outcomes, the historical events, the changes over time, and the motivations and actions of participants and experimenters. Some examples of threats to validity are confounding, mortality, instrumentation, statistical regression, demand characteristics, diffusion of treatments, and compensatory rivalry.
These threats make it difficult to accurately interpret research findings because they can introduce biases or alternative explanations for the observed results. For example, if a study does not control for confounding variables (factors that are related to both the independent and dependent variables), it may be difficult to determine whether the observed relationship between the independent and dependent variables is due to a causal effect or due to the influence of the confounding variable.