Try this 'PICO' method to help you identify and define the scope of each component of your research question.
For each each letter in P. I. C. O. you can find corresponding information using a combination of:
a) background sources listed in the "Encyclopedias, Dictionaries etc." section of this guide, and
b) the results of some simple preliminary CINAHL database searches
In turn, this will help you develop an effective research question (one that is sufficiently specific, focused and answerable), while also identifying appropriate vocabulary to use in your subsequent database searches.
P is for the Patient or the Problem (e.g. a disease or condition) or the Population of interest
I is for Intervention (e.g. therapy, procedure, drug, exposure, test, strategy) or Indication
C is for Comparison (e.g. compared to an alternative intervention or an experimental control)
O is for Outcome (i.e. the consequence, effect or improvement of interest and the measurement thereof)
Not all research questions will fit into the PICO strategy (e.g. questions asking about etiology, risk factors, frequency), so it's helpful also to consider the:
Type of question (e.g. is your question about therapy; diagnosis; prognosis; etiology; harms; prevention)
Type of research study most likely to provide answers (e.g. a systematic review; a randomized controlled trial (RCT); a longitudinal study; a qualitative study)
To incorporate these latter considerations, "PICO" is sometimes extended to PICOTT
PICO is just one of several mnemonic devices or 'frameworks' available to help you develop a clinical research question that is well-formed, focused and answerable. Other frameworks are available that support the development of research questions in other disciplines (e.g. social sciences, business) and for other purposes. For consideration of additional methods--including those serving different research purposes, you might take a look at the extensive collection of frameworks for research questions provided by the University of Maryland Library.