Usability Problems with Zeros on Cluster Maps

Please excuse the lack of post recently, I've been particularly busy with teaching over the last month.

Some Experimental Results: My PhD student Craig has some results from a usability experiment on the UK Crime Maps site.  It's too early in the analysis to discuss in detail but one aspect is coming across very clearly: users are getting confused by the representation of postcodes with no crimes on cluster maps (explanation of cluster maps).

Problem:  When you zoom in on an area in the crime map, you are presented with a view as on the left.  The number of crimes in the last month is shown for each postcode as a number in a blob, e.g. for the postcode mid left (above Thurleigh Rd) there were 4 crimes.  The problem with this visualisation is that the boundaries of the postcodes aren't visible.  If you look on the right I've illustrated roughly where two of the postcodes are with dotted red lines.  The postcode on the left has 1 crime, the one on the right (Devereux Rd) has none and so hasn't got a blob at all.

When users were asked to pick out the postcode with the least crime on the actual crime map (left) they failed to understand that no blob = no crime and identified the blob with the lowest number as the postcode with the lowest crime.

Solution:  On collated point maps zero crime postcodes should be represented by blobs with zeros in them.