People use more than 10 sources of information to arrive at a purchasing decision according to a study by Google. With that many sources of information you can forget about ever being able to quantify every interaction let alone rank the importance of one interaction over another. Compound that with multiple computers (shopping at work, buying at home), multiple devices (searching on mobile, buying on tablet), online and offline (shopping online buying in store) and it’s a big mess. Even if you were able to capture every source there are the seasonal outliers that change customer behavior that your advertising is not responsible for such as seasonal differences, changes in product price, changes in what competitors are doing, and constant finicky consumer taste.
There is no shortage of tools to help you see a portion of the path that a customer uses so that you can quantify the total value of a keyword as an introducer, influencer and closer but all of those labels need to be taken with a big grain of salt.