Lacking Literature?

The more I read theses, proposals and research articles, the more I am questioning the validity of the argument "There is a lack of studies in ..." to justify conducting a new study.

What is the quantity that separates 'lack' and 'sufficient'? Is there a middle category of 'existence' for a given research topic?

Let's say a PhD candidate stated in his 'statement of problem': "There is a lack of study on the psychometric properties of Academic Resilience Scale (ARS) among students in B40 families". If an examiner pointed out that there are at least 2 published studies on the same topic, would we consider the student's statement of problem invalid? Or do we need 3 papers? or 10?

To answer that question, let us consider this simple example. If you want to buy a car that costs RM50k, but you have only RM25k, then you have 'a lack of money' to buy a car. But if your goal or intention is to buy a motorcycle that costs RM10k, then you have more than enough money.

So, I think it is not the quantity of studies that should be prioritized. It is the quality of the existing evidence for a specific research purpose. If there are 2 papers that provide comprehensive evidence of psychometric properties, then we may have sufficient information. If we have 10 papers, but all of them reported only the Cronbach's alpha (not even an index of validity), then we can't conclude that the ARS is a valid measure for use among students from B40 families.

Additionally, how can you justify conducting ONE study to solve the 'lack of studies'? If you're short of RM25k to buy a car, and you proposed a business venture to earn RM10k, would you have solved the problem? Are you solving the WHOLE problem or a small part of the problem?

Lastly, when your study contributes to just a little bit of the problem, PLEASE PLEASE do not overstate the significance of your study. Do not go overboard and claim that your findings would help policymakers and government agencies to improve academic achievement and mental health of students from B40 families. How can one PhD project have that much impact? You solve one small problem, and then claim that a limitation of your study is 'small sample size', do you think policymakers should be impressed with your thesis?

Make sure your thesis does not lack coherence in solving a clearly identified problem.