Last year I ran a highly unscientific experiment. I would regularly put a DVD in an open mail bin in my office to mail it back to Netflix, every late Monday afternoon. I would also count the total number of Netflix DVDs put inside that bin by other people. Over a period of time I observed a continuous and consistent decline in the number of DVDs. I compared my results with the numbers released by Netflix. They matched. I'm not surprised. Even though this was an unscientific experiment on a very small sample size with a high degree of variables, it still gave me insights into the overall real data, that I otherwise had no access to.
Proxies are as useful as real data.
When Uber decides to launch a service in a new city or when they are assessing demand in an existing city they use crime data as surrogate to measure neighborhood activity. This measurement is a basic input in calculating the demand. There are many scenarios and applications where access to a real dataset is either prohibitively expensive or impossible. But, a proxy is almost always available and it is good enough in many cases to make certain decisions that eventually can be validated by real data. This approach, even though simple, is ignored by many product managers and designers. Big Data is not necessarily solving the problem of access to a certain data set that you may need, to design your product or make decisions, but it is certainly opening up an opportunity that didn't exist before: ability to analyze proxy data and use algorithms to correlate them with your own domain.
As I have argued before, the data external to an organization is probably far more valuable than the data that they internally have. Until now the organizations barely had capabilities to analyze a subset of their all internal data. They could not even think of doing anything interesting with the external data. This is rapidly going to change as more and more organizations dip their toes in Big Data. Don't discriminate any data sources, internal or external.
Probably the most popular proxy is the per-capita GDP to measure the standard of living. The Hemline Index is yet another example where it is believed that the women's skirts become shorter (higher hemline) during good economic times and longer during not-so-good economic times.