We recently noticed a problem with the mean accumulated attributes on our point and polygon tilesets. It was first noticeable when we applied styles to display a property with the accumulated mean values.

I created a small set of points to illustrate the problem using public earthquake data from California:
earthquakes_points.zip

-Z0 -z17 -r0 --cluster-distance=5 --use-attribute-for-id=event_id --accumulate-attribute=depth:mean --accumulate-attribute=magnitude:mean

Since this is such a small set of data it's easy to predict what the mean values should be, and the resulting tileset appears to incorrectly generate those values.

Although this is a point feature sample, we noticed the same problem when coalescing polygons.

I have tried the sum, product, max and min functions and they all seam to work. The problem appears limited to the calculated mean values.

Please have a look and confirm what I'm seeing. If this is a bug it'd be nice to get a fix. We have large datasets where the calculated mean value are important properties.