Objectivity means that the researcher has to remain natural while conducting research and to not be bias.
A main disadvantage of longitudinal studies is panel attrition. If you are going to study a group of 1,500 participants once a year then obviously some of these participants will no longer be able to be part of the study. This could be due to illness or death, refusal, or changes in personal information such as contact information and address. Because of this, the same size will get smaller and there wont be as much data to draw conclusions from.
Another disadvantage to longitudinal studies is that because the researcher and participants see each other one a year and over time the participant could subconsciously change their input or information in order to better suit what they believe to be the researchers hypothesis or goal. The study itself can affect how the respondent views and answers the questions.
What Is Meant By The Globalization Of Risk?
What Is Meant by Employer Militancy?
What Is Meant By A Theory Of Mind?
What Is Meant by Group Work?