public static double getDoubleValue( Object o, XLSRecord parent ) throws NumberFormatException
	{
		if( o instanceof Double )
		{
			return (Double) o;
		}
		if( (o == null) || o.toString().equals( "" ) )
		{
			// empty string is interpreted as 0 if show zero values
			if( (parent != null) && parent.getSheet().getWindow2().getShowZeroValues() )
			{
				return 0.0;
			}
			// otherwise, throw error
			throw new NumberFormatException();
		}
		return new Double( o.toString() );    // will throw NumberFormatException if cannot convert
	}