Takt time is the beat of a service – the overall rate at which it needs to function to keep up with demand. Calculating takt time can be tricky if you haven’t done it before. This post describes how to work it out, step by step.
Step One: Work out the service demand
What is the total demand for the service? The unit in which this is measured will depend on the service. For example, I worked with an out-patient department that had over 30,000 appointments a year, so the unit in which they were interested was appointments. In a laboratory service, the unit was blood samples analysed. In an acute in-patient service, their unit of interest was admissions.
There is an important difference between these three examples. Out-patient services are usually pre-booked, while laboratory services and acute hospital services react to demand, which is rarely even: I’ll return to this point later.
Take your overall demand from real figures. In health care, the information is often available electronically, while in other work you have to count the demand from observation or from data collection over a period of time. .
Step Two: Work out the time available to meet the demand
This is the actual time available for the service delivery. So, if the service is open, say, 8am – 6pm, then there are 10 hours, or 600 minutes each working day.
In many cases there are scheduled breaks when the whole service stops, and they need to be subtracted to obtain the true time available. If the service or clinic does not open its doors until 8.30am, and closes for 30 minutes at lunch, then the hour (8am – 8.30am and, say, 12.30 – 1pm) have to be subtracted from the total time available as these times are not available to meet demand. Similarly, if there are scheduled staff meetings during which no other work is done, then these need to be subtracted from the available time.
Breaks can cause confusion. You only subtract breaks if the whole service is closed. For example, if staff members take breaks, but the service remains open for business, then the time of the breaks is not subtracted from the total as the service itself is still open.
Another controversy is around unplanned downtime. In some services, problems with machines mean that, while the time of a problem is unpredictable, the occurrence of the problem at some time over the course of a week or a month is expected. Some people like to subtract this from the total time available.
My preference is to subtract scheduled downtime from the time available, assuming that it affects the whole service, but not to subtract time causes by breakdowns. My rationale for this is that, in a Lean system, you want to build in preventative maintenance. Taking unplanned breakdowns in to account legitimises them, instead of seeing this as vital time lost because of system problems.
Step Three: Divide the time available by the total demand
You have worked out the total service demand, and the time available to meet it. Say, for example, you have calculated that a service has an average demand of 80 new patients a week.
The service is open five days a week, and after subtracting breaks and staff meetings, you work out that the available time is 32 hours, which equates to 1,920 minutes a week. 1,920 divided by eighty is 24 minutes, giving the takt time.
It is usual to work out the takt time in seconds, but this can be hard for people to intepret, so I often convert it back to minutes or hours, depending on the nature of the service. In an out-patient service, for example, the takt time was 3 minutes 35 seconds.
In a service that installed devices in patients’ houses, the takt time was 14 hours. Presenting this in seconds would have added confusion rather than clarification. In a blood science department analysing 3,000 specimens a day, a takt time in seconds may well be useful.
Generally, if making a value stream map, you would not mix measurement currencies. You would not have, for example, 400 seconds as the measurement of one part of the process, but 14 minutes at another – both would either be in minutes or seconds. This makes it easier for the reader to make a direct comparison.
Takt times worked out this way are very useful for looking at services with fairly level demand, or at services over longer periods of time. They can, however, cause some confusion in services with very uneven demand.
Levelling demand is a good Lean principle, but levelling input in to a service is not always under direct control of the service. It can be useful at times to work out takt times for different time periods.
For example, I recently worked with a service that is open 24 hours a day, 365 days a year. Taking their overall annual demand, their takt time is 57 minutes. Looking at visual representations of demand over time, it was apparent that weekends and weekdays were very different. 8am – 8pm weekdays were particularly busy.
Calculating the takt time for the weekends alone, the takt time was over 100 minutes. For the peak hours on weekdays the takt time was 37 minutes. In this case, taking only an average takt time could lead to assumptions about the staffing requirement that would cause problems in improvement work.
Interpretation of takt time
I have discussed this in a previous post, available here. The commonest source of confusion is a worry that takt time describes the time available for each patient, test, or whatever is being measured.
The takt time gives the overall pace at which the service has to work, not the time to be devoted to one person. In theory, the lead time for a process, divided by the takt time, gives the required number of operators.
For example, if an out-patient process takes 30 minutes, and the takt time is ten minutes, the you need 30 / 10 = three staff. In practice, this may underestimate the staff requirements, but it does give you a good place to start.
Photo courtesy of Bernie Condon at freedigitalphotos.net