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When adding tables layers from Excel files, ArcGIS attempts to guess the data-types:

Microsoft Excel does not enforce field types for values during data entry like standard databases do. Therefore, the field type specified in Excel is not used in determining the field type exposed in ArcGIS. Instead, field type in ArcGIS is determined by a scan of the values in the first eight rows for that field. If the scan finds mixed data types in a single field, that field will be returned as a string field and the values will be converted to strings.

Does anyone know what heuristic rules are used here?

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In the past when I've had a requirement to work with Excel data, I've often been able to get the client to agree to using .csv files (which both arcgis and excel handle). –  Kirk Kuykendall Jul 28 '10 at 23:49

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up vote 3 down vote accepted

I'm not sure that this helps with your specific question, and you may well have already seen it, but this is another useful link: http://blogs.esri.com/Support/blogs/esritrainingmatters/archive/2008/11/24/formatting-excel-data-for-use-in-arcgis-desktop.aspx

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The last two sentences provide a pretty good idea on the heuristics they use: scan only the first eight rows for mixed data types, anything mixed is converted into strings. Without further information from ESRI or through lots of testing, we can't say much else about it since we don't have the source code, unfortunately.

The problem with using Excel for 'real analysis' work is that fundamentally its a two-dimensional layout program: data types are not enforced, and the distinction between data and its presentation isn't clearly made.

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+1 for distinction between data and presentation! –  George Jul 29 '10 at 2:54

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