Expression of Uncertainties

According to Dr. S, there are actually uncertainties that I have not considered in my project. Personally, I'm not too sure what is uncertainties.
Dr. S told me to refer to ISO standards on estimating the uncertainties. Then I search on ISO uncertainties on Google. Holly molly, the ISO document is expensive, some of those is over $100.

Then I read this paper. A Comprehensive Comparison of Uncertainty Analysis Tools, and found that there is a guide called, Guide to the expression of Uncertainty in Measurement.

What's better is that the guide is FREE! :D

Here is the link! http://www.bipm.org/en/publications/guides/gum.html

Everything free is good for poor grad student like me.. :)