X Subscripts

In the last activity, we learnt that (X) represents a column of data in your data set. We still need a way to identify single variables in our data set, though.

To be able to represent single variables, we use (X)-subscripts.

For example, using the same list from the previous activity, (X = 30, 21, 59, 45), we can identify each variable in this way:

• (X_1 = 30)
• (X_2 = 21)
• (X_3 = 59)
• (X_4 = 45)

By doing this we can isolate a specific variable, such as the third person in the list’s age: (X_3 = 59)

In the general format, we use the letter (i) as a subscript. It acts as a placeholder for a single unknown variable:
(X_i)

If we are unclear about which variable we want to refer to, we can map out it’s co-ordinates in the data set by referring to the row and column that it is in:
(X_{row, col})
(therefore X_{3, 2} = 59)

NOTE:
When we refer to a position in a spreadsheet, it is always in this order: Row, Column.