Once again you want to go back to your PICO question to determine if the studies you found in your search match your question. You may discover that the studies do not match exactly, and you will have to determine how closely the study matches your question. Sometimes the study will be relevant even if there are other factors which do not match completely.
It might be useful to identify the PICO question addressed in the article by reading the abstract. The abstract is a summary, and you might find it necessary to check the article itself before deciding it's relevance.
Let's look again at our sample PICO question:
In adults with mild depression, does St. Johns Wort compared to prescription antidepressants reduce depressive symptoms?
You find a study that compares the use of several complementary therapies in adults with mild depression. It mentions antidepressants, but it does not compare the effects with the complementary therapies. This study does not meet your criteria, and will not answer your PICO question. You decide not to look at the full article.
Another article does compare St. John's Wort with prescription antidepressants, but the patient population is specific to adults who have recently lost a spouse, and the length of time for the study was quite short, only one month. It does, however, look at the ability to cope with the loss of a loved one, and decrease in depressive symptoms. As you are not working with bereaved adults and your patient group does not match the study group exactly, this research might provide some relevant data to answer your question, and you decide to look at the full text of the article.
After selecting your relevant studies from the search, you can decide whether you want to adjust your search. After reading the articles, you may find some references at the end of the articles that will lead to find additional sources.