Final answer:
The external validity problem most likely impacting the original study when different results are found through replication is related to how well the original study's design and measures represented the concept it intended to measure (validity). Factors influencing the study's external validity could include the experimental conditions, testing environment, or participant characteristics.
Step-by-step explanation:
When researchers replicated their experiment using a more holistic measure of learning and found that their results did not match the original study, the external validity problem most affecting the original study would likely be related to how well the study measured what it was designed to measure, which is referred to as validity. Validity issues can arise if the measures used in the study do not accurately capture the intended concept, or if the study context does not reflect the real-world scenario it intends to represent.
Additionally, several factors such as participant characteristics, the testing environment, or the specific conditions under which the experiment was conducted could affect the external validity and lead to different outcomes in replicated studies.
Ensuring a study's reliability, which represents how likely research results are to be replicated if the study is reproduced, is crucial for the credibility of research findings. Replication enhances reliability and provides evidence that the results are not due to chance. When external validity is compromised, the ability to generalize findings to other settings, populations, or times is limited, raising questions about the usefulness and applicability of the original research findings.