A process improvement team has been working on a project to reduce coding errors for the past $15$ months and the data for the number and type of coding errors can be downloaded from the link provided. Use this data to answer the question below.

The team implemented several solutions that took effect after month $4.$ Use an appropriate control chart to assess process behavior over the $15$ month time span. Is there evidence of an improvement?

A The data indicate that the process is stable over the $15$ month time period and that the improvement actions were effective.

B The process varies, but is statistically stable; observed improvements are due to common cause variation, so there is no true improvement.

C The process is unstable over the $15$ month timeframe, and there is no discernible trend in the number of coding errors.

D There is an apparent improvement trend, but the sample size is too small to draw any meaningful conclusions.

E The process is unstable over the $15$ months, and displays a downward shift in the average errors$/$claims$,$ indicating a favorable shift.