# How to normalise data properly without a maximum / minimum constant?

I use a stock screener for investing purposes. When I'm trying to filter stocks, I can use a factor like Price-to-Book (share price divided by book value per share), Price-to-Sales, etc to rank stocks and get the Top Decile.

I want to combine several factors with an equal weight, and for this I need to normalise these factors to values between 0-1. Why? If I did not normalise it, averaging a Price-to-Book Value of 12 (in a sample ranging from 12-16) with a Price-to-Sales of 3 (in a sample ranging from 1 to 4) would obviously hugely skew the average because the Price-to-book would lift the average enormously despite being at the lower end of the sample, while the Price-to-Sales would reduce it despite being at the higher end of the spectrum.

How do I currently normalise? If I want to normalise Price-to-Book (PB), I calculate, for every stock:

("PB of current stock" - minimum PB of sample) / ( max of sample - min of sample )


The minimum and maximum values of the sample need to be looked up manually by me in the dataset right before normalising, because they will be different in a month (stock's fundamentals change all the time) and for every calculation I can only access the characteristics of the current stock being looked at, I cannot ask for the average of the dataset or the min / max programmatically. So to properly normalise the data, I have to look up the minimum and maximum values every time I want to normalise.

How can I use a future-proof normalisation process where I do not need to lookup the max and min constants manually every time for every factor? Is there a mathematical way to do so? Or a smarter way of normalising the data without the need for a min / max constant?