Minitab identifies a column as either text, numeric, or date/time based on the first value you enter in the column. If the first value you enter in a column is not a numeric or date/time value, Minitab identifies it as a text column and adds the suffix, "-T" to the column heading (for example, C1-T).
Even if you clear text characters from a text column, Minitab will continue to treat it as text.
To change the data type from text to numeric:
1. Choose Data > Change Data Type > Text to Numeric.
Note that numeric data will be recognized as text if data is imported from another application and Minitab places the column names in the first data row of the Minitab worksheet. To ensure that the data is imported correctly, use Options and Preview in the File > Open Worksheet dialog box. If a column is appearing as text when you preview the worksheet, you can change the format to numeric by choosing Numeric from the "Type" drop down menu for that column.