Classifying rows of data by the number of days to close
In addition to Heslacher's review, I'd also move the initial if-check to a separate function:

private bool IsRowInvalid(DataRow row)
{
    return row["total_daysclose"] == System.DBNull.Value 
           || row["closed_in0to3_count"] == System.DBNull.Value
           ||row["closed_in4to7_count"] == System.DBNull.Value 
           || row["closedGtr7_count"] == System.DBNull.Value 
           ||row["Closed"] == System.DBNull.Value;
}


This method you then call like this:

foreach (DataRow row in dt.Rows)
{
    if (IsRowInvalid(row))
    {
        continue;
    }

    // other logic here
}


I would also rethink names like "0to3" and "total_daysclose" if possible: these naming styles are inconsistent and IMHO bad; I'd much prefer "ZeroToThree" and "TotalDaysClose". "Gtrthan7" is even worse, since it contains an abbreviation and I really can't see any reason why.

Note that I would also move column names to constants, perhaps even move those to a static class, e.g.

internal static class ColumnName
{
    public const string AverageDays = "AverageDays";
    public const string Closed = "Closed";
}


I didn't implement this in the method above because it was too much work.



I just noticed your edit, and I'm wondering whether the NULL checks are even necessary, since I don't see any NULLs in your sample output. I also agree with the comments that this code indicates issues elsewhere, and that simply focusing on this method is not enough.