My point of view is that, first you need to now which type of articles are you gathering, I mean, you want to look for a specific method, technique, performance scores and so forth?
For example, if you want to summarize the quantity of research work made in one specific area you should look for every method. On the other hand, maybe you want to explain the main techniques made in Deep Neuronal Network for a specific classification or prediction process, so in that case maybe you need to find all the work related with that problem but JUST with DNN. Finally, you may want to present the performance of one type of system (maybe DNN), so maybe you should divide the articles between different performances like accuracy, sensitivity or specificity.