Traditionally, law enforcement has created its own failing point, which is lack of supervisory preparation. In most agencies, the training of an entry-level supervisor comes after their promotion. You are flying by the seat of your pants for months, if not years. Finally, the coveted supervisor training class opens, and then you find out what it is all about. This lack of succession planning is a major issue that has daily, if not hourly, effects on a department.
A novel idea would be to offer this training to those about to take the promotion test. Results would be immediate with a knowledgeable candidate pool, quicker start-up, and then not having to "undo" poor decisions made due to a lack of proper training. Granted, there may be some who after this view into the liability infused and pressurized environment of supervision may pass on it. The bottom line is, when potential supervisors receive training at the outset, everybody wins. Even if some candidates do not get promoted at that time, they will possess deeper insights about the bigger picture.
I will be the first to admit that promotions are not fair and that people often get promoted because of a "lucky day" taking the test. Still several agencies cling to the age of seniority-based promotions. The issue I have with this is that just sitting in a garage for 20 years doesn't mean you will become a master mechanic nor turn into a Buick. In other words, sometimes you need a new or fresher look at leadership and not the career curmudgeon.
For those who aspire to become supervisors, your preparation for the test and process started the day you walked across the police academy stage. Those who wait to read the announcement for sergeant testing and only then start preparations will probably fail. Read the requirements ahead of time, as some may take some time and effort to meet. As I told my son, who was recently promoted in the U.S. Air Force, you start on the next pay grade the day you get the current one.
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