We've read the data and computed summaries in two separate steps. In some cases, we may want to create the summaries while reading the initial data. This is an optimization that may save a little bit of processing time. We could write a more complex input reduction that emitted the grand total, the shift totals, and the defect type totals. These Counter objects would be built one item at a time.
We've focused on using the Counter instances, because they seem to allow us flexibility. Any changes to the data acquisition will still create Counter instances and won't change the subsequent analysis.
Here's how we can compute the probabilities of defect by shift and by defect type:
from fractions import ...
