Sometimes it helps to be able to view a variable across time or space. In our case, the years of departure of the slaving voyages are mixed together, and it would be helpful to see how the numbers of voyages, or, better, the number of people being enslaved changed over the century or so we have data for.
This can be achieved using the SPSS function which aggregates the data against another variable called the ‘break variable’. If you imagine, in our case, each year being a box, the break variable is ‘yearam’, the year of departure, and each box is a year.
To create an aggregate calculation, choose Data and then Aggregate.
The Aggregate dialog box will open and you can move the data you are interested in (‘number’ and ‘yearam’) into the appropriate boxes using the right arrows.
Notice that ‘number’ is given as the mean of the variable. We actually want the sum of the variable (the total number of people per year, not the mean number per voyage in a year). This can be changed by clicking on Function; the Aggregate Function dialog opens.
Choose Sum from the Summary Statistics radio buttons and click Continue. You will return to the Aggregate dialog box.
Here, given that part of the function of the Aggregate operation is to re-order our data, we need to ensure that the output goes to a new data set. This is done from the Save area on the Aggregate dialog box.
Click on ‘Create a new dataset…’ and enter a name for it. Then click OK.
SPSS will do the calculation and open a new data set with the results. If you click on this (it will initially appear as a third window in the SPSS icon in your system tray) your new dataset will open.
Durham SPSS 