Getting to the truth, the ground truth, and nothing but the ground truth.

Takeaways for learning from HCOMP 2019, Part 2

At HCOMP 2019, there was a lot of information about machine learning that I found relevant to building educational technology. Surprisingly to me, I didn’t find other ed-tech companies and organizations at the Fairness, Accountability, and Transparency conference I went to last year in Atlanta or the 2019 HCOMP conference. Maybe ed-tech organizations don’t have research groups that are publishing openly and thus they don’t come to these academic conferences. Maybe readers of this blog will send me pointers to who I missed! 

Mini machine learning terminology primer from a novice (skippable if you already know these): To train a machine learning algorithm that is going to decide something or categorize something, you need to start out with a set of things for which you already know the correct decisions or categories. Those are the ‘ground-truths’ that you use to train the algorithm. You can think of the algorithm as a toddler. If you want the algorithm to recognize and distinguish dogs from cats, you need to show it a bunch of dogs and cats and tell it what they are. Mom and Dad say —  “look, a kitty”; “see the puppy?” An algorithm can be ‘over-fitted’ to the ground truth you give it. The toddler example is when your toddler knows the animals you showed them (that Fifi is a cat and Fido is a dog), but doesn’t know what new animals are, for example the neighbor’s pet cat. To add a further wrinkle, if you are creating a ground-truth, it is always great if you have Mom and Dad to create the labels, but sometimes all you can get are toddlers (novices) labeling. Using novices to train is related to the idea of wisdom of the crowd, where the opinion of a collection of people is used rather than a single expert.  You can also introduce bias into your algorithm by showing it only calico cats in the training phase, causing it to only label calicos as “cats” later on. Recent real world examples of training bias come from facial recognition algorithms that were trained on light-skinned people and therefore have trouble recognizing black and brown faces. 

Creating ground truth: A whole chunk of the talks were about different ways of creating ‘ground truths’ using ‘wisdom of the crowd’ techniques. Ed-tech needs quite a bit of ground-truth about the world to train algorithms to help students learn effectively. “How difficult is this task or problem?” “What concepts are needed to do this task/problem?” “What concepts are a part of this text/example/explanation/video?” “Is this solution to this task/problem correct, partially correct, displaying a misconception, or just plain wrong?” 

Finding the best-of-the-crowd: Several of the presentations were about finding and motivating the best of the crowd. If you can find and/or train ‘experts’ in the crowd, you can get to the ground-truth at lower cost (in time or money). I am hoping that ed-tech can use these techniques to crowdsource effective practice exercises, examples, solutions, and explanations. 

  1. Wisdom of the toddlers. Heinecke, et. al ( described a three step method for obtaining a ground truth from non-experts. First, they used a large number of people and expensive mathematical methods to obtain a small ground truth. (If we are sticking with the cats and dogs example from the primer above, you have a large number of toddlers tell you whether a few animals are cats and dogs and use math to decide which animals ARE cats and ARE dogs using wisdom of the toddlers.) From there, step 2 is to find a small set of those large numbers of people who were the best at determining a ground-truth, and use them to create more ground-truth. (Find a group of toddlers who together labeled the cats and dogs correctly, and use them to label a whole bunch more cats and dogs). Finally, you use the large set of ground truth to train a machine learning algorithm. I think this is very exciting for learning content because we have students and faculty doing their day to day work and we might be able to find sets of them that can help answer the questions above.
  2. Misconceptions of the herd: One complicating factor in educational technology ground truths is the prominent presence of misconceptions. The Best Paper winner at the conference, Simoiu et. al (, found an interesting, relevant, and in hindsight unsurprising result. This group did a systematic study of crowds answering 1000 questions from 50 different topical domains. They found that averaging the crowd’s answers almost always yields significantly better results than the average (50th percentile) person. They also wanted to see the effects of social influences on the crowd. When they showed the ‘consensus’ answer (current three most popular answers) to individuals, the crowd would be swayed by early wrong answers and thus did NOT perform on average better than the average unswayed person. Since misconceptions (wrong answers due to faulty understanding) are well known phenomena in learning, and are particularly resistant to change (if you haven’t seen Derek Muller’s wonderful 6 minute TED talk about this, go see it now!) we need to be particularly careful not to aid their contagion when introducing social features.

Are misconceptions like overfitting in machine learning? As an aside, my friend and colleague Sidney Burrus told an interesting story that sheds light on the persistence of misconceptions. Sidney talked about how, during the initial transition point between an earth-centered and sun-centered model of the solar system, the earth-centered model was much better at predicting orbits, because people had spent a lot of time adding detail to the model to help it correctly predict known phenomena. The first sun-centered models, however, used circular orbits and did a poor job of prediction, even though they had more ‘truth’ in them ultimately. Those early earth-centered models were tightly ‘fitted’ to the known orbits. They would not have been good at predicting new orbits, just like an overfitted machine learning model will fail on new data. 

1 thought on “Getting to the truth, the ground truth, and nothing but the ground truth.

  1. Pingback: Leveling up crowd-sourced educational content — with a little help from machine learning and lessons from HCOMP19. | Kathi Fletcher

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