# Setwise Comparison: a faster, more consistent way to make judgements

I originally wrote this post in 2016 for the Sparrho blog.

Have you ever wondered whether doctors are consistent in their judgements? In some cases, they really aren’t. When asked to rate videos of patients with multiple sclerosis (a disease that causes impaired movement) on a numeric scale from 0 being completely healthy to 4 being severely impaired, clinicians struggled to be consistent, often giving the same patient different scores at different times, and disagreeing amongst themselves. This difficulty is quite common, and not unique to doctors — people often have to assign scores to difficult, abstract concepts, such as “How good was a musical performance?” or “How much do you agree or disagree with this statement?” Time and time again, it has been shown through research that people are fundamentally inconsistent at this type of activity, no matter the setting or level of expertise.

The field of ‘machine learning’, which can help to automate such scoring (e.g. automatically rating patients according to their disability), is based on the method that we can give the computer a set of examples for which the score is known, in the hope that the computer can use these to ‘learn’ how to assign scores to new, unseen examples. But if the computer is taught from examples where the score is inconsistently assigned, the result is that the computer learns to assign inconsistent, unusable scores to new, unseen examples.

To solve this problem, we brought together an understanding of how humans work with some mathematical tricks. The fundamental insight is that it is easier and more consistent for humans to provide preference judgements (e.g. “is this higher/lower/equal to that?”) as opposed to absolute value judgements (e.g. “is this a 4 or a 5?”). The problem is, even if you have as few as 50 items to assign scores, you already have 50 x 49 = 2450 ways of pairing them together. This balloons to nearly 10,000 comparisons when you have 100 items. Clearly, this doesn’t scale. So we scale this using a mathematical insight: namely, that if you’ve compared A to B, and B to C, you can guess with reasonably high accuracy what the relationship is between A and C. This ‘guessing’ is done with a computer algorithm called TrueSkill, which was originally invented to help rank people playing multiplayer games by their skill, so that they could be better matched to online opponents. Using TrueSkill, we can reduce the number of comparisons required by a significant amount, so that increasing the number of items no longer results in a huge increase in comparisons. This study has advanced our understanding of how people quantify difficult concepts, and has presented a new method which balances the strengths of people and computers to help people efficiently and consistently provide scores to many items.

### Why is this important for researchers in fields other than computer vision?

This study shows a new way to quickly and consistently have humans rate items on a continuous scale (e.g. “rate the happiness of the individual in this picture on a scale of 1 to 5”). It works through the use of preference judgements (e.g. “is this higher/lower/equal to that?”) as opposed to absolute value judgements (e.g. “is this a 4 or a 5?”), combined with an algorithmic ranking system which can reduce the need to compare every item with every other item. This was initially motivated by the need to have higher-quality labels for machine learning systems, but can be applied in any domain where humans have difficulty placing items along a scale. In our study we showed that clinicians can use our method to achieve far higher consistency than was previously thought possible in their assessment of motor illness.

We built a nifty tool to help clinicians perform Setwise Comparison, which you can see in the video below: https://www.youtube.com/watch?v=Q1hW-UXU3YE

### Why is this important for researchers in the same field?

This study describes a novel method for efficiently eliciting high-consistency continuous labels, which can be used as training data for machine learning systems, when the concept being labelled has unclear boundaries — a common scenario in several machine learning domains, such as affect recognition, automated sports coaching, and automated disease assessment. Label consistency is improved through the use of preference judgements, that is, labellers sort training data on a continuum, rather than providing absolute value judgements. Efficiency is improved through the use of comparison in sets (as opposed to pairwise comparison), and leveraging probabilistic inference through the TrueSkill algorithm to infer the relationship between data which have not explicitly been compared. The system was evaluated on the real-world case study of clinicians assessing motor degeneration in multiple sclerosis (MS) patients, and was shown to have an unprecedented level of consistency, exceeding widely-accepted clinical ‘gold standards’.