One of the common trends in machine learning has been an emphasis on the use of unlabeled data. The argument goes something like “there aren’t many labeled web pages out there, but there are a huge number of web pages, so we must find a way to take advantage of them.” There are several standard approaches for doing this:
- Unsupervised Learning. You use only unlabeled data. In a typical application, you cluster the data and hope that the clusters somehow correspond to what you care about.
- Semisupervised Learning. You use both unlabeled and labeled data to build a predictor. The unlabeled data influences the learned predictor in some way.
- Active Learning. You have unlabeled data and access to a labeling oracle. You interactively choose which examples to label so as to optimize prediction accuracy.
It seems there is a fourth approach worth serious investigation—automated labeling. The approach goes as follows:
- Identify some subset of observed values to predict from the others.
- Build a predictor.
- Use the output of the predictor to define a new prediction problem.
- Repeat…
Examples of this sort seem to come up in robotics very naturally. An extreme version of this is:
- Predict nearby things given touch sensor output.
- Predict medium distance things given the nearby predictor.
- Predict far distance things given the medium distance predictor.
Some of the participants in the LAGR project are using this approach.
A less extreme version was the DARPA grand challenge winner where the output of a laser range finder was used to form a road-or-not predictor for a camera image.
These automated labeling techniques transform an unsupervised learning problem into a supervised learning problem, which has huge implications: we understand supervised learning much better and can bring to bear a host of techniques.
The set of work on automated labeling is sketchy—right now it is mostly just an observed-as-useful technique for which we have no general understanding. Some relevant bits of algorithm and theory are:
- Reinforcement learning to classification reductions which convert rewards into labels.
- Cotraining which considers a setting containing multiple data sources. When predictors using different data sources agree on unlabeled data, an inferred label is automatically created.
It’s easy to imagine that undiscovered algorithms and theory exist to guide and use this empirically useful technique.