Sample Size Help for Observation Study

I wanted to calculate sample size for a single group, observational study. I'll be using a questionnaire to predict outcome (i.e. score on another questionnaire) in a sample of participants but I don't have a comparator or control group. How would I calculate sample size for this type of study design? Thanks!


Omega Contributor
How is the variables formatted, what is your tentative sample size or pool to draw from and most importantly do you have a hypothesis. If so, what is it???
I'm looking at whether scores on a self-report questionnaire about sleep predict functioning (based on total score of another questionnaire). I'm able to draw for a pool of approximately 400 people.