How can I compare two (email) lists without viewing the content

Andy July 3, 2011

I need to compare an email list with a list curated by someone else and see the % of overlap.

The first thought is to make hashes out of each email. However this only works until a match is found. Once we have overlap we will both know who is a double-subscriber, and this is arguably confidential info.

Any idea how to do this.

Is there any service that would allow us to compare? I am thinking that the matches would be meaningless for that 3rd party.

  1. Tina
    July 10, 2011 at 10:32 pm


    did Joseph's solution work for you? Please let us know!

  2. Joseph Gorman
    July 4, 2011 at 4:14 am

    If the lists are in excel format (or can be opened in excel), here's a quick way (2007+). First, select both lists. Second, go to Conditional Formatting -> Highlight Cells Rules -> Duplicate Values. You can select the cells based on their formatting in the full find/replace dialog if you want to or just look at the cells.

    Source: (also contains a method to do this in excel 03).