Final answer:
Volunteer bias is most serious in observational research. Longitudinal research involves tracking a group over time, and observer bias is managed in observational studies by establishing clear criteria for recording behaviors and ensuring inter-rater reliability. The correct option is b.
Step-by-step explanation:
Volunteer bias is most serious in observational research, where the participants who choose to be part of the study may not represent the larger population being studied. This type of bias can significantly affect the reliability and generalization of the research findings.
Longitudinal research involves following a group of research participants for an extended period of time, observing and recording behaviors or conditions, and is also susceptible to volunteer bias, as the willingness of participants to stay involved over time could influence the results.
Observer bias in observational research refers to the issue where researchers may unconsciously record observations that support their expectations or the project's goals. To minimize this, it's crucial to establish clear criteria for recording behaviors and often includes measures such as inter-rater reliability to ensure consistency across different observers.
Hence, Option b is correct.