Number of observations used for calculating STD

[HyunJee]

I have a dataset which contains multiple blood pressure measurements per patient. The issue is, that some patients may only have 3 blood pressure measurements while other will have 10 blood pressure measurements. I want to calculate the standard deviation of all a patients blood pressure measurements within a given month - does one patient having only 3 versus another patient have 10 blood pressure measurements in a month affect how I will interpret the STD of blood pressure per month?  If I have not explained this clearly, please let me know. Thank you.

[Rick_SAS · Accepted solution]

The standard deviation is already adjusted for the number of observations. That's why it is called the "standard" (-ized) deviation (from the mean).  The only thing that you need to be aware of for interpretation is that there are two ways that someone can have a big standard deviation: (1) they can have moderate variablity but a small number of observations (such as 3), or (2) they can have a lot of variability with a large number of observatios (such as 10).