Category Archives: 4.1-Work and Rest

Sequence Constraints

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The constraints on sequences of shifts are used to control the different types of shifts that employees would work consecutively. For example, if employees share the night shifts and all have to work them, you may need to control the maximum number of consecutive night shifts.

 

Sequence constraints can also apply to mandatory sequences. For example, it is mandatory to have 2 days off after 3 night shifts. You may also have forbidden sequences where employees can’t work a day shift after an evening shift.

 

In short, sequences control the different combinations of shift types a planner can use to build a schedule. These controls are usually in place for both fairness and fatigue. When an employee works on all shifts, any schedule they get will be difficult and full of mistakes from the employee’s perspective.

 

Planners need to protect themselves by stating clearly what sequences are controlled, for fairness and/or fatigue.

 

Horizon Constraints

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As you can see, we started with a shift, then to a day, moved on to a week, and we could then move on to bi-weekly, and so on. These constraints can be combined into one single concept.

 

If you think of time as one long endless axis, a week is simply a delimitation of that axis, or a chunk of that axis of time. The day that we just discussed earlier is exactly that: a chunk of 24 hours on the axis of time. So we could apply exactly the same logic to any time span we know of like the week, the month, the year, etc.

 

Therefore, this section is called horizon constraints. The horizon represents that chunk or span of time that the constraints will be measured against.

 

A horizon is determined by three important aspects:

  1. The span: the length of the horizon (a week, a month, a specific number of days, etc)
  2. The offset: when does that span start on that axis of time? Is it Monday, Sunday? If it’s longer than a week, which date do you start counting? You need to determine a date or a day that says: this is where we start counting.
  3. The step: this one is a little tricky. The step is the value to add to the offset to get to the next time span to validate. In most cases, you don’t need to think about this one because once you’ve defined your span to be a week for example, you simply count on that week and never overlap the next week (the step has the same value as the span). But in some industries like transportation, safety comes first. Therefore, there is a maximum number of worked hours that is measured against a ten-day period, but that is for any ten-day period. That means that if a planner looks at work from February 1st to February 10, the schedule would show an employee working say 50 hours. The planner also needs to look at the amount of work from February 2 to February 11 and make sure the sum of work still does not exceed the maximum. That would therefore be a constraint that has a horizon of 10 days and a step of 1 day.

 

It’s important not to confuse the definition of a constraint, or how you count the constraint, to the time span it applies on. For example, if you have a minimum number of weekends you need to count, the time span is not the weekend. The time span is how long you measure the weekends for. The weekend is in the conditions that make the constraint count or not.

 

Examples of Horizon constraints:

Maximum of 80 hours of work on a bi-weekly basis: In this case, the span is 14 days, the offset is a date in the past that is a Sunday or Monday (depending on your week start), and the step is 14 days.

Maximum of 140 hours of work per month: Span is monthly, offset is a date in the past that is usually in the first days of the month. You don’t want to count a monthly thing starting on the 29th of a month because it gets confusing for a planner when February comes along. Best practice is to pick the first of the month when you count monthly constraints.

Minimum of 5 days of rest any 25 days of work: Span is 25 days and the offset could actually be any day in the past since the constraint says ‘any 25 days’. The planner will have to measure all spans of 25 days across the schedule both when creating it as well as maintaining it.

 

Week Constraints

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A week needs a frontier just like the day has one. Usually, the time you have selected for the day will be the same for the week; you therefore only need to pick the day that starts the week (which in most cases is either Sunday or Monday).

 

Ideally, your week definition will be the same as the week on which you pay on. That is just to keep things simple. The most important thing for employees after their schedule is their pay check so if it’s easy for them to follow both the schedule and the pay check together, all the better for everyone. But the pay week and the schedule week are actually two totally separate concepts. The schedule week is the one actually driving things like overtime pay and bonus pay. The thresholds on overtime are and should be counted based on the scheduled time span. This means that if you schedule from Sunday to Saturday, nothing is stopping you from counting everything based on that week, and then pay based on a Thursday to Wednesday week.

 

You may wonder who in their right mind set would do that. Well again, it depends on the business. Some scheduling constraints are based on a 10-day pattern and overtime is calculated based on that 10-day time span. As you can imagine, the employees are not paid every 10 days. This happens in areas where 24/7 coverage is required and the schedule is split in standard cycles (see chapter 5) that are not dividable by 7 (a standard week).

 

You’ll also need to establish the definition of regular days on and regular days off. Just like we explained on how to associate a shift to a day in order to count hours, you also need to define what is a day on and what is a day off so that you can eventually count the constraints that are about days.

 

Most of the time, a day on is a day with an associated shift and a day off is one that has no associated shift. If you have selected to divide the hours of shifts that overlap your day frontier, then you may decide that a day on is a day that contains regular worked hours.

 

Some even elect to differentiate depending on when the shift was scheduled. For example, someone schedule to work Monday to Thursday on a day shift will have 4 days on and 3 days off in the week at the time of posting the schedule. If extra work comes along and you add a shift to that person on Friday, this shift is scheduled on a regular day off and would be subject to premiums including overtime even though the employee still has not reached 40 hours. It is simply because that employee was not planned to work. So the Friday, even though has a shift, remains a day off.

 

Although the example sounds weird, it exists. And the planner has to keep different definitions of days on and days off and be very clear on which one applies to which constraint. In these cases, the planner would be very precise in the definition of each constraint to explain how it is counted against both the hours, the occurrence, and the publication time.

 

The weekly constraints are therefore about sums of daily constraints and also about sequences of events that occur in the week:

  1. Minimum hours in a week: As mentioned in chapter 3, employees may be hired with a minimum hours of paid hours in a week. Note that this minimum is not of hours worked, but hours paid and therefore include vacation or other time-off.
  2. Maximum hours in a week: This constraint can be split in two. There is the maximum number of regular hours and the maximum number of hours overall. A 40-hour full time employee will have a maximum number of hours of 40 hours. That’s easy. But when you need to schedule that employee in overtime, what becomes the real maximum where an employee starts being burned out? 55 hours? 72 hours? There is a second maximum that is the sum of all hours worked regardless of how they are paid. That second maximum usually applies after the schedule is posted when employees are getting called for overtime shifts (even though sometimes, overtime can be scheduled).
  3. Number of shifts per week: The number of shifts worked per week is usually constrained by a maximum. Some industries offer a guaranteed minimum amount of shifts instead of a minimum number of hours, but most are counting shift occurrences to control a maximum.
  4. Maximum number of consecutive days on: As you would imagine, someone can’t work every day on and on with no day off. This maximum can be interpreted in different ways. It can be the number of days on scheduled at the time of posting and excluding overtime. It can also be the number of days on overall, meaning that the week is not a boundary. If the employee works Friday and Saturday this week, and also Sunday and Monday next week, that is a total of 4 consecutive days that span over two weeks. Some industries keep the maximum to one week and do not look at previous or next weeks. In that case, the example above would only show 2 consecutive days.
  5. Minimum number of consecutive days off: All the constraints we have seen so far are about things happening. Days off occurrences are about things not happening. When nothing happens, you count the day off based on the definitions you established earlier. This constraint of giving a minimum number of consecutive days off (most likely 2 days off) is mostly for employee satisfaction. We usually enjoy a weekend, but for people who work shifts, there is no fixed weekend necessarily. Therefore, trying to give them consecutive days off is one way to increase employee satisfaction.

 

Day Constraints

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Before listing the constraints of a day, we must define what a day is. A day, for most of us, starts at midnight and ends at midnight. Then the date changes and we are on another day. That’s the easy definition.

 

Some businesses that need to stay open 24 hours a day 7 days a week will have a different definition of the day. For example, a casino has its peak of activity at midnight which makes things difficult to break down if a planner needs to split the hours when most employees are scheduled. In these cases, the split of the day happens at the lowest point of business in a day. Again in a casino, this would be at 5am or 6am. Hospitals and manufacturing also measure and count schedule constraints against a day that fits standard shifts. This would typically be at the end or the beginning of the night shift. For example, if the night shift is from 11pm to 7am, you either count that shift to be part of the day it starts (making the day be from 7am to 7am) or to the day it ends (making the day be 11pm to 11pm and making one hour of the previous day belong to the day).

 

Note: try not to make that frontier 2am or 3am. These hours are when time changes occur twice a year in most areas of the world. Having a frontier at 2am will make things confusing when that 2am becomes 3am when daylight savings time kicks in. You’ll feel like an hour has disappeared into oblivion.

 

Once you’ve established what the day is, you also need to determine how the shift is tied to that day. If your constraint says that employees can work a maximum of 12 hours in a day, how do you handle the shift that overlaps that day frontier? Let’s say that day frontier is midnight and your employee is scheduled for an 8-hour shift starting at 11pm. There is one hour on the day the shift starts and seven hours on the day the shift ends. Therefore, you need to establish the rule on how hours are counted for the employee. In most cases, all hours will be counted towards the day the shift starts, but I’ve even seen businesses determine the day of a shift based on where does the middle of the shift falls. For example, a shift from 7pm to 3am would belong to the day it starts (the middle being 11pm), but a shift from 9pm to 5am would belong to the day it ends (the middle being 1am).

 

Now that you have established how to count, we can take a look at what to count:

  1. Minimum hours in a day: Although it may sound like the same constraint as the minimum shift duration, it is quite different. First, it depends on how you count hours in the day. Second, you may have split shifts. A split shift is when an employee works a few hours in the morning and a few hours in the afternoon. Typical employees working split shifts are bus drivers where they work during rush hours. So although you have a minimum duration of 3 hours on a shift, you may have a minimum duration of 6 hours on a day for example. So if you schedule a 3-hour shift, you need to schedule a second one on the same day to respect that daily constraint.
  2. Maximum hours in a day: For the same reasons, and again depending on how you count your hours, you may have to define a maximum hours per day constraint. This is also different than the maximum shift duration constraint since split shifts may come into play (see above).
  3. Minimum rest between shifts: This constraint is the most common. How much rest time do I need to give an employee before scheduling them for another shift? This minimum rest is the duration between two shifts where you expect them to go home (this is not the separation time between split shifts). You need to establish a proper value based on the average shift duration you normally schedule and the expected travel/rest time anyone would require before being back and productive. The longer the shift duration is, the longer the minimal rest should be. You may also have different minimum rest values depending on the shift that is worked. For example, after a night shift, you may force a 12- hour rest period instead of the standard 8 hours. The same goes to have a different rest period apply before a shift. If the day shift is particularly busy, you may have a 10-hour rest period before the shift is started to make sure the employees come in rested and productive.
  4. Maximum number of shifts per day: This constraint will be required depending on how you associate shifts to a day. If you schedule an 8-hour shift starting at 7am and another one for the same day starting at 11pm, you have two shifts starting on the same day with an 8-hour period in between. You have respected the minimum rest rule but the employee would end up being awake for 24 hours in a row basically (not much time to go home eat, sleep and come back). Also for split shifts purposes, you may define how many splits may occur in a day by having a maximum number of shifts per day.
  5. Shift separation: Shift separation is for split shifts. As explained above, a split shift is sometimes required by the business simply because of the workload profile that has multiple peaks in the same day. Although these are good for the business, they are usually not very popular with employees. With this type of shift, you need a reasonable minimum and maximum duration between the two parts of the shift. The minimum value is driven by the business. If the minimum is too low, then the separation becomes one long break that the employer would have to pay. The maximum value is driven by employees. If the maximum is too high, then employees are away from home for a very long duration. Thus, a reasonable balance is required. Some employers pay up to 25% the duration of the shift separation in order to be able to schedule them and not end up with outright refusals from employees. This all depends on the business requirements and demographics that are involved.

 

Shift Constraints

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First things first: what is a shift? The definition of the word shift will change according to the environment you are in. For some, it represents the span of a day where everyone is scheduled to work at the same time (the day shift in a manufacturing shop). For others, a shift represents the start and end time of continuous work presence for one employee. In this section, the shift means the latter: a continuous work presence that has a commitment from the employee to be ready for work until the end of the presence.

 

The constraints surrounding are measurable either in time or in occurrences:

  1. Minimum shift duration: When an employee comes in to work, their presence will be for a minimum time. You would not ask someone to come in just for 15 minutes to clean the floor and then go home. You’ll have to determine the best minimal shift duration that makes sense for your business (some are as low as 3 hours for part-time employees).
  2. Maximum shift duration: Obviously, you can’t ask someone to be present at work for eternity. They will eventually have to go home and sleep. Although some environments schedule employees for up to 36 hours (doctors in a hospital have very long shifts), this maximum rarely surpasses 12 hours. Some businesses give high bonuses when the shift needs to exceed 12 hours. Other areas are controlled by law (for example truck drivers can’t drive for more than 10 hours in a row) and impose constraints for public safety. All in all, you’ll need to establish a maximum.
  3. Maximum duration before a break: An employee will require a coffee break at some point. So within a shift, you need to define how long someone can work without a break. Note that breaks are not necessarily scheduled.
  4. Minimum break duration: A two-minute break won’t do. There is a minimum amount of break is required which is usually around 10 to 15 minutes.
  5. Maximum duration doing the same activity: Some activities are harder to do than others. In order to reduce the strain on employees and also reduce the risk of injuries, some organizations have established maximum durations for the same activity. This applies mostly to manufacturing areas where some jobs are physically demanding.

 

The constraints listed above are imposed by the fact that humans need rest and common sense requires keeping them safe from injury and productive for the business.

 

Other constraints about the shift can be driven by the business to keep the employees productive:

  1. Maximum break duration: In order to keep employees productive, you need to split the total break time into multiple breaks. Having long breaks have showed that employees come back less productive and take more time to get back up to their regular productivity level. How many times have you gone out for a long lunch and come back feeling drowsy, no matter what you ate?
  2. Maximum number of different activities in a shift: There is a limited number of things an employee can do within a day. If too many activities are scheduled, that employee will become non-productive at some point. The mental change required to start a new activity makes that activity non-productive for the first minutes until the brain gets into gear. Too many activities simply multiply this non-productive time.
  3. Minimum activity duration: Although this constraint could be seen as reaching the same goal as the previous one, it’s actually listed for a different reason. An activity that requires a long time to setup or longer preparation time will be constrained by a minimum duration. You wouldn’t want to repeat that long non-productive time too many times so you would want to leave an employee on that activity as long as possible.

 

Work and Rest

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People have a limited capacity of work.

 

Although we all have different limits, the employee has a certain capacity for work within a certain frame of time. This frame is expressed at different resolutions, just like the workload. For example, an employee can’t work more than 3 consecutive hours before getting a break. That same employee can’t work more than 7 consecutive days before getting a day off. It is the same rest constraint, but expressed at a different resolution (hours vs. days).