Yes, and thanks for all your suggestions but I think I've now made a decision.

I had started with 2 files, call them file1 and file2. After a few false starts and suchlike I eventually got to 2 combined files. One was formed by exporting/importing individuals added after a certain date from file1 to file2. The other by exporting/importing from file2 to file1.

For some reason these were not identical (ref numbers aside). Insight helped solve some of the problems, as did exporting a whole file and importing it into a new one on a couple of times.

But I was still left with one combined files claiming more citations than the other.

But last night I checked. File1 started with 262 citations (wherever they are). File2 started with 262 citations. One of the combined files has 262 citations, the other has only 257, implying it has lost some data.

I am going to go forward with the 262 combined one and hope it is correct.

Thanks again