Dan:
Thanks for your help. I looked over the original excel data and discovered that N/A was entered whenever no data was collect. When I imported the data into ArcMap, the software, which, I just learned, scans the first eight records in order to determine the data type, found a mix of data types. In these cases it defaults to text. So the result was a sprinkling of text data types among the majority of double data types.
So I went into the original excel data and deleted the N/A, leaving these cells blank, and imported the data again. This time the excel files merged fine.
Thanks again for helping me with this.
Mel