Looking for a current business issue to illustrate statistical process control when you are covering Supplement 6? The Wall Street Journal (Sept.24,2012) notes how “American Airlines continued to rack up high numbers of flight delays and cancellations, blaming a dispute with its pilots union. The union, meanwhile, denied that pilots disrupted flights unnecessarily.”
Can the same set of data be used to make opposite points in an argument? It’s not that statistics lie, it is more in how we present all of the available data points, as can see in this timely example regarding the alleged “sick out” of American Airlines pilots. Here is a 13 month “snapshot” of percent of pilots out sick at American that you can use in class: 9/18/11, 4.9% ; 10/18/11, 9.5% ; 11/18/11, 5.0% ; 12/18/11, 6.5%, ; 1/18/12, 5.4% ; 2/18/12 , 6.6% ; 3/18/12, 6.6% ; 4/18/12, 6.0% ; 5/18/12, 7.0% ; 6/18/12, 7.4%; 7/18/12, 6.1%; 8/18/12, 6.6%; and 9/18/12, 7.5%. (The number of pilots dropped a few percent during this period in American’s financial struggles, from a high of 7,840 to a current 7,563.)
“By my calculations,” writes a Dallas Morning News(Sept. 20, 2012) reporter, “the number of pilots on sick leave was 45.7% higher on Sept. 18, 2012, than on Sept. 18, 2011, up 177 pilots. That seems like an increase in sick leave usage”. (See the bar chart graph above used to make this point). American’s spokesman adds that sick leave “has been up more than 20 percent year over year and has been elevated for months.”
Counters the union: “Contrary to claims by management, we have confirmed that pilot sick rates have not deviated from historical norms”. (Here we see the SPC p-chart showing percent of sick days being within p-chart control limits).
What happens to the p-chart if the 1st two months last year are excluded? Ask your students to recompute the control limits and draw conclusions.