Final answer:
Cohort studies are expensive, time-consuming, and often face the issue of participant dropout, all of which affect the study's feasibility and validity.
Step-by-step explanation:
The disadvantages of a cohort study include the fact that they can be expensive and time-consuming. This is because such studies follow a group of similar individuals over an extended period to observe outcomes, which requires substantial financial resources and may span several years or decades. Furthermore, there is a high chance of participants dropping out due to various life changes like moving, illness, or even loss of interest, which is referred to as attrition. This dropout can affect the representativeness and validity of the findings. Additionally, all these factors contribute to the difficulty in conducting such studies, and researchers must account for these challenges when planning and executing a cohort study.