Final answer:
A study can lack validity if replication is insufficient, biased, or constitutes pseudo-replication, which does not truly reflect independent sampling units.
Step-by-step explanation:
A study can lack validity when repeating it if there are issues with insufficient or biased replication, and if pseudo-replication occurs. To ensure that a study is valid, it is essential to have true independence among sampling units. Otherwise, the study may be susceptible to Type II errors due to downwardly biased estimates of variance, leading to an inability to detect real effects or differences that exist.
Additionally, studies on human behavior and other sociological topics must also factor in that people may not always provide accurate responses due to various reasons such as lying, misremembering, or giving socially desirable answers.
Furthermore, the integrity of experimental design is key for obtaining reliable and valid results. This involves proper random assignment to treatment groups, the use of control groups, and blinding of researchers and subjects wherever possible. A poorly designed study will fail to produce data that genuinely reflects the treatment effect, thereby compromising the study's validity. Ethical constraints also limit the types of experiments that can be conducted, impacting the scope of study designs available to researchers.