This is another question I often get, and it is a good one. Gone are the days when you could locate all of the studies in your field and go through them as an individual. You could probably spend the rest of your life searching and reading and never be done. So knowing when you have read enough studies is one of many decisions you need to make as the author of your review. So how do you make it?
There are two things you need to consider when you are deciding if your literature review is done. As I have noted in a previous post, your literature review is an argument in which you make the case for why your research question needs to be asked. So have you made your case? Do you take the reader, step by step, through the patterns and trends you have discerned in the body of work on your topic, establishing how these patterns and trends add up to the need to ask your research question? Have you instanced each of these patterns and trends with findings from the studies you've read? Sit down with your draft and read it honestly and critically. Is the draft really just an annotated bibliography in paragraph form (this study said A; that study said B, etc.)? Or do you have a series of encyclopedia entries (descriptive accounts of studies on particular topics)? Or have you written a chronological account that has very little to do with your actual research question? If the answer to any of these questions is 'yes', you aren't done yet. This doesn't mean that what you have written so far is wrong or bad or was a waste of your time; rather, it simply means you have come to grips with the first step: getting your head round what each of the studies you have read established or claimed. You do need to know this in order to write your review. But you also need to go on to the next step: understanding why all of these findings and claims are significant when it comes to establishing why your research question needs to be asked.
You can think of the argument you are making in your literature review as a framework you are building. Think of this prompt: My research question needs to be asked because . . . . . How do findings from previous studies in your field allow you to complete this prompt? For example, your research question might be about how students understand the concept of criticality. As part of your literature review, you might collect and synthesize a number of different ways that criticality has been defined in the academic literature. When you first do this, your argument will change every time you read a new paper, because it is still developing. Every new paper you read gives you new information that you have to take into account, so you adjust your framework. For example, you might have been arguing that your research question needs to be asked because we do not have good definitions of criticality. But after reading a number of papers, you realise that actually we do have good definitions of criticality, they just stress different things, so you have to change your argument. Eventually, though, your argument will come together. You will know why your research question needs to be asked, and you will have solid evidence for that from previous studies in your field. And you will read a paper that does not tell you anything new because it is just another example of a point you have already made and supported with numerous other examples from the literature. So now you have one more example you can add to an already robustly supported point. And then you read another paper, and it is yet another example of this same point that is now even more robustly supported. And so on. And this brings us to the second thing you need to consider when you are deciding if your literature review is done: saturation. Saturation is another concept from qualitative research. When all of your data fits satisfactorily into the framework you have built to explain it, you have reached saturation. Applying this to your literature review, you reach saturation when all the new papers you read fit comfortably as yet other examples of points you have already robustly supported. Now you know that your literature review is ready. Will someone else come along and read that one paper you didn't get to that contradicts one of your robustly supported points? Maybe. That can be the start of their dissertation. Should you check your judgement with your supervisor? Certainly. But if you have a solid argument for why your research question should be asked and none of the additional papers you have been reading tell you anything new, you know you are in good shape.
