In a within-subjects experiment, which of the following accurately describes a practice effect?

A practice effect is a specific type of carryover effect in within-subjects experiments where participants perform a task better in later experimental conditions. This improvement occurs not because of the experimental manipulation, but simply because the participants have gained experience and had the opportunity to practice the task during earlier conditions.

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DePaul University

1.  Mturk workers are not a representative sample
2. Workers may be more willing to cheat
3. Practice effects
4. Less likely to get a random sample

Cons for using MTurk as participant pool

A carryover effect is a specific type of order effect that occurs when a participant's behavior or response in one experimental condition is directly influenced by having been tested in a prior condition. When these effects are not the primary focus of the research, they can become problematic confounding variables, making it difficult to determine whether changes in the dependent variable are due to the independent variable or merely the lingering impact of previous conditions.

Carryover Effect

Paolacci, G., & Chandler, J. (2014). Current Directions in Psychological Science, 23(3) 184-188. https://doi.org/10.1177/0963721414531598. 

Inside the Turk: Understanding 
Mechanical Turk as a Participant Pool

https://kpu.pressbooks.pub/psychmethods4e/

KPU Research Methods in Psychology - 4th American Edition

Mturk can be used as a supplement for choosing participants but the workers are not a representative sample of the population as a whole, because of location, demographics, and personality. Most Mtukers are in the United States and India, are usually around 30 years old, are overeducated, underemployed, and liberal. They are also more introverted and neurotic than college aged participants. 

Not a representative sample

Mturkers may be more likely to cheat on evaluations compared to college students, due to lack of monitoring and social desireability. As of this paper, there was not a way to remotely monitor participants as they completed tasks, on Mturk. This may give workers more incentive to look up answers on the Internet. Likewise, workers are more likely to exhibit social desireability personalities than college students. Workers want to both please the researcher and qualify for better paid studies. This means they are more likely to choose responses they think will please the researcher, rather than their true opinions. 

More likely to cheat

According to this article, “about 10% of workers are responsible for completing 41% of tasks” on Mturk. Higher rated participants are more likely to qualify for studies that are more complex, even though they may not qualify for these studies. Because workers are more likely to choose tasks they are more familiar with, we may not gain a truly random sample for our study.

May not get a random sample

Practice Effect

A fatigue effect is a specific type of carryover effect where participants perform a task worse in subsequent experimental conditions because they have become physically or mentally tired, or bored, from participating in earlier conditions. If the order of conditions is not properly controlled, this decline in performance can serve as a confounding variable that obscures the true effect of the independent variable.

Fatigue Effect

A context effect, or contrast effect, is a specific carryover effect where being tested in one condition alters how participants perceive stimuli or interpret their task in later conditions. For example, an average-looking individual might be judged more harshly if participants have just finished evaluating a highly attractive person, demonstrating how the sequence of exposure skews subsequent judgments.

Context Effect

When deciding between experimental approaches, researchers generally prioritize the within-subjects design if practical. A good rule of thumb is to opt for a within-subjects experiment—utilizing proper counterbalancing—if there is sufficient time to test each participant in all conditions and if there are no serious concerns about confounding carryover effects. If these conditions cannot be met, researchers should consider a between-subjects design instead.

Rule of Thumb for Experimental Design Choice

In a taste-testing experiment, participants first drink a highly sweetened, strongly flavored soda and then immediately taste a mild, lightly flavored sparkling water. If the intense sweetness of the first drink temporarily dulls their taste buds and alters how they perceive the sparkling water, what is this confounding variable called?

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