I studied sleep quality by PSQI on reporting the results which is better to report the results as continuous data ( global PSQI score ) or categorical data ( normal and abnormal sleep quality )
On using the categorized form what is global PSQI score cut of point that could be considered
Why not try both ways？ In my opinion，the categorized PSQI is with more clinical relevance， while the continuous score may help to quantitatively reveal the relation between sleep quality and other factors interested. So if you could only choose one， I suggest to make the decision depending on your aim.
I would suggest using a categorical variable, such as "optimal" (PSQI < 6), "borderline" (PSQI 6-7), and "poor" (PSQI >7). See Lund et al. (2010), which uses these categories. If you use the global score as a continuous variable, you will get too large of a range to analyze the data. For example, poor sleepers can have a PSQI of 19, which would be considered an outlier using a continuous variable. You could always just use "poor" or "optimal/normal" as 2 categories, if you are sure to explicitly operationalize your variables.
Outlier is not an enemy for the analysis of continuous variable. First, before analysis the outliers should be examined carefully. If the value is not reasonable, it should be excluded, no matter in the analysis of continuous variable or categorized variable. Second, only a small proportion of the result should be outlier, or no outlier at all. So this is a problem about the quality of the data, not the style of the data.
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