threats to internal validity
There are some threats to internal validity: specifically design confounds, selection effects, and order effects. In the following examples, determine which, if any, of these threats exist. Explain your answers. As part of the answer, it should make sure that you define the particular threat. One or more example may not have any threats (that is, it may be a well designed study, with strong internal validity); if that is the case, state that.
1. An applied psychologist wants to test the effectiveness of an intervention to increase awareness of the environmental impact of disposable water bottles. The initiative involves asking students to use a smartphone to track the number of disposable water bottles they use, categorizing whether they reuse them, throw them away, or recycle them. He asks for volunteers in a large geology course. Students must own a smartphone in order to participate in the intervention group. Fifty volunteers who owned a smartphone were assigned to the tracking condition (they downloaded a free smartphone app for this purpose). Fifty more students who were interested, but who did not own a smartphone, made up the comparison group. This group was simply exposed to a short video on the impact of plastic water bott les. At the end of a two- week period during which the students in the0020smartphone group tracked their water bottle use, the researcher found that students in the tracking program were more likely to have purchased a reusable water bottle in the past week compared with the students in the comparison group. He concluded that his smartphone tracking program raised awareness, causing students to purchase reusable bottles.
2. A cognitive psychologist believes that people learn better when they spread out their studying over several days, so she creates a study with three groups of participants. Each group studies the same list of 120 Chinese vocabulary words (none of the participants had studied Chinese before). One group studies the words for 20 minutes on the first day. Th e second group studies the words for 20 minutes on the first and second days. The last group studies the words for 20 minutes on the first, second, and third days. On the fourth day, all of the participants are tested on how well they have learned the Chinese vocabulary words. The people in the last group scored the best, so the researcher concludes that distributed studying does improve people’s ability to learn.
3. A human factors psychologist is comparing visibility features for automobiles. (Human factors psychologists study how humans interact with the material world.) He plans to test whether drivers will avoid obstacles behind their cars more effectively when the car is equipped with an enhanced rear- view mirror, a rear video camera system, or an object detector that sets off a buzzer alarm. He places a sample of 25 drivers into each of three cars. The cars are identical except for their object detectors. Th e drivers spend 1 hour familiarizing themselves with their vehicles and their object detectors by running through a set of drills on a closed driver’s course. During a test phase, the researcher places a set of objects behind each driver. The test objects range in height, color, and movement. Each driver attempts to back up his or her car while avoiding each of the test objects. Each object is presented three times each. The psychologist finds that, on average, drivers respond more accurately to the rear camera video system compared to the rear-view mirror or the buzzer alarm.