user25645: I'm looking for metrics that quantify how similar/different two positions are.
I have found a paper that used pawn structures to search for similar games in a database. This one tries to use reachable squares, as well as attack and defense structures do achieve this.
Metrics can be arbitrary, but I wonder if other approaches exist.

user11567: Ultimately, what is considered 'similar' depends on what one is trying to achieve by measuring similarity. I suspect, a common chess engine use-case would be to try predicting if the two positions are going to return same evaluation result.
Here are a few brainstorm-style ideas:

Compare lists of legal moves and measure the degree to which they
overlap.
Base the measure on the number of pieces whose positions are mismatched
Base the measure on the minimum number of moves needed to get from
position A to position B.

Some food for thought: https://en.wikipedia.org/wiki/Edit_distance

25645: <p>I'm looking for metrics that quantify how similar/different two positions are.</p>
<p>I have found a paper that used pawn structures to search for similar games in a database. <a href="http://doras.dcu.ie/20378/1/ganguly-sigir2014.pdf" rel="nofollow noreferrer">This one</a> tries to use reachable squares, as well as attack and defense structures do achieve this.</p>
<p>Metrics can be arbitrary, but I wonder if other approaches exist.</p>
 11567: <p>Ultimately, what is considered 'similar' depends on what one is trying to achieve by measuring similarity. I suspect, a common chess engine use-case would be to try predicting if the two positions are going to return same evaluation result.</p>
<p>Here are a few brainstorm-style ideas:</p>
<ul>
<li>Compare lists of legal moves and measure the degree to which they
overlap.</li>
<li>Base the measure on the number of pieces whose positions are mismatched</li>
<li>Base the measure on the minimum number of moves needed to get from
position A to position B.</li>
</ul>
<p>Some food for thought: <a href="https://en.wikipedia.org/wiki/Edit_distance" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/Edit_distance</a></p>
