Number of cash registers is still an incomplete description. We should ask what wait time if acceptable. We should also find out if we are talking about a mix of self checkout or employee managed cash register. What about cash registers for customer with less than 10 items?
If the plan is to only hire 5 people for cash register, then we are locked with a maximum of 5 cash registers. Here also, we are assuming that there will be 5 people available for the entire day. These points are worth clarifying upfront.
There are different cash register needs based on time/day of week.
Let’s start by mapping out a user’s journey in case of a employee managed cash register.
> User waits in line
> User arrives at employee managed cash register
> User unloads items
> Employee/User scans items
> User pays for items
> User completes checkout
A single cash register can do the job.
It would just make the customers wait longer and longer.
So if we have 2 customers in checkout line at any given time.
And each of them take 5 minutes to complete their transaction. Then with one cash register you could say that if they both arrived at the same time, then the wait would be roughly 5 mins before they are served, and we have solved our problem.
This means that we only need to know how many people will arrive in checkout at the same time. And, we simply put half of those in terms of cash registers.
So, if 20 people arrive at the same time, then we need 10 cash registers.
A visit to a Walmart store with comparable neighborhood would give some guidance.
We should also visit some nearby grocery stores to get some data to help with the analysis.
Let’s say we get the following numbers.
> Nearby Walmart, 50 miles away, wait time = 5 to 20 mins, 20 customers at the same time, 10 checkout registers.
> This Walmart neighborhood serves 20,000 people in their 5 mile circular neighborhood.
> People hate the wait time
> People have started to go to other grocery stores.
> Customer satisfaction is very low.
Now let’s build our model a bit with a goal of reducing the wait time.
> In our immediate 5 mile radius, we will be serving about 20,000 people as well.
> By looking at the traffic for the other Walmart & other stores, we could see that on average we have 20 customers in checkout at any given time. So putting 10 cash registers, would give each customer a 5 minute delay, which would seem to work.
> However, this solution does not account for the fact that there will be peak traffic which is when wait time could double.
> Therefore for peak traffic, when we have 40 customers in checkout, we should open up additional cash registers. So, we should need 20 cash registers.
10 is our min.
20 is our max.
Now 20 does seem a bit high. If you visit at Walmart, they never have more than 8-10 employee managed checkout stores.
Now we think again about measuring how much time it takes one person to go thru checkout if they have less than 10 items vs those that have more than 10 items.
Let’s put some number assumptions here.
If a buyer has less than 10 items, then they would take about 2.5 minutes.
For greater than 10 items, they take about 5 minutes.
Now, let’s break down the number of active people in our line by saying that we have 20 that have greater than 10 items. And, another 20 that have more than 20 items.
For the people that have greater than 10 items, we still have the same number of registers = 10.
For people that have less than 10 items, we would have less number of registers = 5.
So, now we are down to 15.