The probability of a rise or fall in blood pressure?

The probability of a rise or fall in blood pressure??

I have developed a database program that searches for all blood pressure readings that are similar to the readings of any given patient. The program then retrieves the following day’s data from all the matching patients to find out what (if any) the probability of the patient being analysed will have a higher or lower blood pressure and what the likely percentage increase/decrease will be.

To do this, I have simply counted the number of patients that have a rising/falling blood pressure and averaged the rise or fall to give a percentage e.g. 14 out of 20 patients blood pressure had risen and therefore the probability of this patient’s blood pressure being raised tomorrow with be 70% and it will raise by approximately 5.63% i.e. that average raise of these 14 patients.

I’m not too sure whether this (never studied statistics nor probability theory) is correct or not and would be most graceful if anyone can give me some help and advise.

Kind regards and thanks in advance for any help

Laurence