The first threat is history, which entails how external factors or events of the study affect the research’s outcomes when they occur due to time passing. Thus, the specific occurrences between the first and second measurements threaten internal validity. The second threat is maturation, which entails the effects of the subjects’ developmental stages, such as psychological or biological, due to the passage of time. For example, suppose the number of clients visiting the emergency department due to asthma can decrease due to reduced incidence of the disease in childhood, not because of any treatment program. Therefore, maturation and history can affect the study’s internal validity.
The third threat is instrumentation since a study’s outcomes are affected by how accurate an instrument and its measurement are. This becomes a threat when researchers use different measures in the post-test and pre-test phases. The fourth threat is testing, and it occurs when the outcomes of the post-test are affected by the pre-test. For example, given a performance test, the participants’ productivity will be higher in the post-test because a familiar procedure was used in the pre-test. Thus, instrumentation and testing are threats to internal validity.
The fifth threat is statistical regression since, after many repeated tests, extreme scores usually cluster around the mean. This usually occurs when extreme scores have been used to select subjects. For example, if high-stress scores were used to recruit subjects for a study, regression would affect the research outcomes. The sixth threat is selection biases because when participants cannot be assigned to groups at random, selection biases arise, threatening internal validity. The threat originates from the fact that the groups are not equal in terms of the variables under consideration.
The seventh threat is attrition; participants can be lost, resulting in unequal comparisons of groups due to differential loss. The operational procedures used and the research design chosen determine the internal validity’s confidence level. The eighth threat is selection interactions since. Sometimes, a selection threat can interact with other threats. For example, participants from different groups can compare notes and either feel resentful of others or discover the study’s aim, which will affect the outcomes of the study. Thus, attrition and selection interactions affect internal validity.