Your first model

Build a small clinic check-in model, run it, and use it to test a second desk. You'll see a queue appear, find out why, and put a number on the fix.

The example

Patients arrive at a clinic about every 5 minutes. One desk checks them in, which takes 4 minutes. Your question: should the clinic add a second desk?

You will stay in the Basic view, which is where new users start. You need two shapes and one arrow.

Step 1: Draw the process

1. Draw a rectangle and label it Patients Arrive.
2. Draw a second rectangle and label it Check-in.
3. Draw an arrow from Patients Arrive to Check-in. Make sure it attaches to both shapes.

Step 2: Convert the diagram to a model

In the side panel, click Convert Diagram to Model. Quodsi reads the shapes and reports what it found: 1 Generator, 1 Activity and 1 Connector. Patients Arrive becomes the generator because work only leaves it. Check-in becomes the activity because work arrives there.

Press Ctrl+Z if you want to undo the conversion.

Step 3: Set the run length

Click empty canvas so nothing is selected. The panel shows the Model editor.

1. Set Model Name to Clinic Check-in.
2. Set Run Length to 8 hours, one working day. New models start at 24 hours.

Step 4: Set the arrivals

Select Patients Arrive. The panel shows the Generator editor.

1. Leave Type on Rate.
2. Under Inter-arrival Duration, set Distribution Type to Exponential.
3. Set Mean to 5 and the unit to minutes.

Inter-arrival means the gap between one arrival and the next. Exponential makes each gap random, with an average of 5 minutes. Some patients come close together and some gaps are long, which is how independent arrivals behave.

Step 5: Set the processing time

Select Check-in. The panel shows the Activity editor.

1. On the Basic tab, check that Capacity is 1. That is one patient at a time, so one desk.
2. Open the Actions tab. Check-in has one step, which reads Wait for followed by a time.
3. Set the time to 4 minutes.

Click empty canvas again. The chip at the top of the Model editor shows ✓ when the model is ready. A ⚠ with a number means there are errors to fix. Click it to see them.

Step 6: Run your Baseline

1. Click What-if Analysis. A window opens on the What-if Runs tab.
2. Click Create Baseline scenario. Quodsi starts the run right away.
3. When the run finishes, click Results.

Step 7: Read the results

Overview shows the headline numbers. Now open Flow Analytics and find Where's the constraint? It ranks each activity. With only one activity, Check-in is at the top. Constrained by says what limited it. Verdict says how it kept up with demand.

Look at the time patients spent in the system. The check-in itself takes 4 minutes, so anything above that is waiting.

Step 8: Try a second desk

1. On the Check-in row, under What if?, click Study. New what-if opens with a change to the capacity of Check-in already filled in.
2. Check that the value is 2, then click the create button at the bottom of the window. It reads Create scenario or Create study.
3. Run it. Click Run in the window that asks you to confirm.
4. When it finishes, open its results and compare it with the Baseline.

What you should see

We ran this model on the Quodsi engine for one desk and for two. Each ran 8 hours with 10 replications, which are repeats of the same day with different random draws. New models use 10. Your numbers will differ a little because the random draws differ, but the pattern should match.

Three things are worth noticing:

• Throughput barely moved. The clinic sees about the same number of patients either way. The desk wasn't limiting volume. It was creating waiting.
• Time in system fell by about two thirds. The arithmetic said the desk had enough capacity. The simulation shows what that capacity feels like to a patient.
• The spread collapsed. With one desk, a bad day was four times as long as a good one. With two desks, the days look alike.

Is a second desk worth it? That depends on what it costs and what a shorter wait is worth to the clinic. Quodsi gives you the numbers for that conversation. To add cost, open the Financial tab on an activity and tick Enable financial tracking. It is in the Basic view.

Things to try

• Change the arrivals to a Mean of 4 minutes, so work arrives as fast as one desk can handle it. See what happens to the queue.
• Change the Mean to 3 minutes. Work now arrives faster than one desk can finish it, and the queue keeps growing until the run ends.
• Switch the Actions tab to Classic and, under Processing Time Duration, change the Distribution Type from Constant to a random time that still averages 4 minutes. The workload is the same, but the waiting changes.