We've been talking about all the things that we will learn in this course, but so far, we just talked from the comfort of my office. Let's get out into the real world. A little while ago, one of the interventional radiologist here at Penn, had asked me to look at their patient flow. But what do you look for when looking for patient flows? Well imagine, you and I will take two folding chairs, coffee, donuts, and make ourselves comfortable in front of Penn's Department of Interventional Radiology. We also take a notebook and while I'm eating my donuts you're going to take detailed notes. Ready? So here we go. At 7:35 AM the first patient shows up, patient goes in, door is shut. That is all that we'll see for now. I ask you to create a graph tracking the cumulative inflow of patients in blue, and the cumulative outflow of patients in red. Ten minute later, we're now at 7:45, the next patient shows up, and at 8:10 the third patient comes. Before the fourth person shows up at 9:30 AM, however the first patient, the guy who'd came at 7:35, comes out of the department. At 10 AM, I ask you a test question, how many patients are in the unit? Sharp as you are, you would respond, three, because four went in and one came out. From then onwards, I stop asking stupid questions, and we just take our notes. Only patient five surprises both of us as he got in and out of the department in 15 minutes. We later learned that he refused to sign the consent form. At the end of our day in the hospital, we have built a graph that looks something like this. Again, the blue graph captures the cumulative inflow, and the red graph is accumulative outflow. These two graphs meet at the time the patient has left the unit. First observe that we have taken care of 11 patients today, we call this number the flow rate. The flow rate measures how many patients we handled in a given period of time, in our case, one day. We can also think of a patient flow per hour, about one patient flows through the process per hour. But there a couple of more things this graph teaches us about the patient flow. We can also compute the number of patients that are in the department at any given moment of time. This is simply the difference between the inflow curve and the outflow curve. The number fluctuates between 0 in the morning 4 around 3 PM, we call this number the inventory. Sorry, I know this sounds really disrespectful to all patients, but that's just the word. Inventory is the number of patients in the process. Finally, we can also see how long each patient spent in the process. Some patients spent a long time, as did patient nine, for examples. Other get in and out much faster, recall patient five thst was in and out in 15 minutes. We call this number the Flow Time. The flow time measures how long it took the patient to flow through the process. So let's review our new definitions. To be general, we'll extend these definitions to include other things that might flow through a process, not just patients. So rather than talking about the patient as our atomic unit of analysis, we'll just speak about a generic flow unit. The following definitions work for all processes and operations. For example, we could think about the flow of specimens through our pathology lab, or the flow of insurance claims through our billing department. So, for a generic flow unit we define flow rate, oftentimes also called throughput, as the number of flow units going through the process per unit of time. In our case it was 11 patients per day, 200 calls per hour in our call center, 30 insurance claims per day in our accounting department. It always is flow units per unit of time. Flow time is the time it takes a flow unit to flow through the process from beginning to end. When dealing with patients, we often times refer to this number as the length of stay. But flow time could be about 20 minutes that our caller spent on hold in the call center, it could be 3 months that patients wait for their bill. And then there is inventory. Inventory is the number of flow units in a process at any given moment in time. Again, I apologize for the term, when it comes to patients, oftentimes people talk about the census, inventory is a more abstract word. And I argue that this abstract word will help us in the remainder of this course. Looking at any process is overwhelming, lots of stuff goes on, and for us as observes it's really easy to get overwhelmed. So we have to be very analytic about the process flow. The three process measures we introduced today are the three most important measure in any process analysis. They themselves will not tell you how to improve the process, but will if we don't understand these three measures, we really don't understand the process. So I will hereby promote these three measures to the most important operational metrics of this course. You notice that none of these measures has any money attached to it now. We will worry about money later, flow comes first. Taiichi Ohno, of the Toyota production system, allegedly said product has to flow like water. Now let me reword this. Do patients have to flow like water? We want to keep things moving, and I'd like to think of hospitals as places where patients flow to a healthier state of life. Once we understand flow rate, inventory, and flow time, we can move on to the next step. We can ask ourselves, why these measures are what they are. Why don't we take care of more patients? Why does it take so long? And what are all these patients doing right now here in my waiting room? Lots of more questions we need to answer, but that's for another day.
