WHAT IS SLACK AND WHY IS IT NEEDED?

What can you do when you need to get home as quickly as possible during rush hour? You could choose an alternate route on your phone’s map app to avoid traffic after checking different route options before leaving. This example of flexibility illustrates the concept of slack, which depends on people’s ability to adapt and be flexible when facing uncertainty.

Slack is a means to deal with uncertainty and variability in a planned or opportunistic way by using different types of resources. Slack does not only imply the use of extra or idle resources. Existing resources can be repurposed for a different use to cope with uncertainty and variability. Slack also creates opportunities for innovation by encouraging organizations and people to experiment with new strategies (Bourgeois, 1981; Lawson, 2001).

Buffer is the best-known type of slack, being defined as a cushion to protect processes against variability. Buffers are used to address known, existing variability, helping to achieve a desired level of outcome (Alves and Tommelein, 2003; Buchmann-Slorup, 2014). However, other types of slack may be used to deal with other types of uncertainty or organizational demands.

From an organizational perspective, slack provides the resources required to meet demand or take actions at a strategic level (e.g., innovating and establishing coalitions). From a production system perspective, slack is necessary because of the complex nature of construction projects; accordingly, construction projects should be regarded as complex socio-technical systems (CSSs).

The complex nature of construction projects - or other CSSs - can be described by their structural complexity and uncertainty (Williams, 1999). Structural complexity arises in systems that have many interrelated parts. The degree of complexity is associated with the number and uniqueness of parts and the extent of their interrelationships. Uncertainty is related to project goals (i.e., how well-defined the goals are) and means (i.e., how well-defined the methods of achieving those goals are). Uncertainty is also often defined as a predictable form of uncertainty. However, uncertainty may be defined broadly as a state of unknowing in which there is a lack of knowledge of a situation.

In CSSs, much of the uncertainty is caused by human and social influences or organizational conditions that make the performance of a system difficult to predict and control. Complexity makes it difficult for people to understand, describe, and manage systems. Furthermore, highly complex systems or projects produce outcomes that are emergent, i.e., outcomes that are difficult to predict in advance.

Figure 1 presents a concept map showing connections between concepts investigated during the development of this research study (see Formoso et al., 2021). The map is divided into three zones: (i) why is slack needed? (antecedents); (ii) what is slack? (classification and development); and (iii) which are the impacts of slack? (consequences).

Figure 1: Slack concept map (from Formoso et al., 2021)

HOW TO IMPLEMENT SLACK

Inventories, time, equipment, people, money, and information are some examples of slack resources. These resources can be strategically used to provide CSSs with:

1. Redundancy: additional resources (in excess) that are made available – e.g., redundant procedures or functions.
2. Flexibility: resources that can be used in different ways – e.g., multi-skilled workers, multi-purpose equipment, or adjustments by the workforce to deal with the unpredictable nature of situations.

Secondary slack strategies include margin of maneuver, which is a combination of redundancy and flexibility based on the autonomy of individuals or groups of people, and work-in-progress (WIP), which is a type of redundancy. While margin of maneuver determines how resources can be reshaped according to necessary and existing conditions, WIP (a type of slack frequently observed in construction projects) is manifested in the form of inventories of unfinished products or backlogs of available workspaces that are often used as a mechanism to cope with the lack of flow reliability.

WHAT ARE THE IMPACTS OF SLACK

In CSSs, predicting emerging events is a challenge, and slack can be regarded as a key risk mitigation strategy. Construction project risks are often accounted for in a fragmented way in managerial processes such as procurement, design, quality, safety, and production planning and control. Production teams also carry out informal risk management in everyday work when making decisions to assess trade-offs and prioritize the allocation of resources – e.g., when considering the risks of working overtime to complete an activity.

The main positive impacts of slack are described below. These impacts affect the performance of construction projects by improving productivity, generating more value for clients, and reducing project duration.

1. Resilience: intrinsic ability of a system to adjust its functioning prior to, during, or following events such as changes, disturbances, and opportunities, so that it can sustain required operations under expected or unexpected conditions (Hollnagel et al. 2015). Reliability: ability of a system and its components to perform required functions under stated conditions for a specified period of time (Rausand and Høyland, 2004).
2. Robustness: ability of a system to maintain functionality when exposed to a variety of external or internal conditions and disturbances. Robustness is observed whenever there exists a sufficient repertoire of actions to counter perturbations (Saurin, 2017).
3. Flexibility of output: adapting products to fulfill specific customer requirements without incurring high transition penalties or large changes in performance outcomes (Petroni and Bevilacqua, 2002). It is strongly related to the mass customization strategy.

From a Lean Production perspective, a negative impact of slack is the manifestation of waste through activities that take time, resources, or space but do not add value from the customer’s perspective. Some types of slack are considered waste when in excess, such as inventory of materials, work-in-progress, and unproductive time.

Eliminating waste is a driver for improvement, allowing problems that represent improvement opportunities to be identified. Therefore, slack as a potential source of waste should be measured and eliminated consistent with the Lean philosophy of continuous improvement.

EXAMPLES OF SLACK

Daily huddles
Strategy of deployment: redundancy.
Resource: people (problem-solving perspective).

Why and how is slack implemented? Daily huddles are meetings to exchange information or discuss the current production status and activities that need to be carried out. These meetings are very relevant in constantly changing environments in which unexpected events emerge. Two types of slack can be found: (i) double checking existing conditions to perform tasks (e.g., safety measures, availability of resources), and (ii) considering different perspectives in decision making, as no single actor is usually capable of devising and implementing effective solutions.

Figure 2: Daily huddle (from Saurin et al., 2021)

Reallocation of workers to meet daily production goals
Strategy of deployment: flexibility.
Resource: people (both the number of workers and their problem-solving perspective).

Why and how was slack implemented? In the construction of an airport terminal, the conclusion of a certain construction phase after the deadline was subject to contractual penalty. Workers were reallocated to other tasks to compensate for delays. These decisions were made and communicated during daily huddles.

Figure 3: Reallocation of workers (from Fireman and Saurin, 2020)

If you are interested in learning more about slack, please take the time to read the two papers listed below. All references used in this post are also included in these papers:

Formoso, C., Tommelein, I. D., Saurin, T. A., Koskela, L., Fireman, M., Barth, K., Bataglin, F., Viana, D., Coelho, R., Singh, V., Zani, C., Ransolin, N., & Disconzi, C. (2021). Slack in Construction – Part 1: Core Concepts. In Proceedings of the 29th Annual Conference of the International Group for Lean Construction (pp. 187-196). IGLC. https://doi.org/10.24928/2021/0183

Saurin, T. A., Viana, D. D., Formoso, C. T., Tommelein, I. D., Koskela, L., Fireman, M., Barth, K., Bataglin, F., Coelho, R., Singh, V., Zani, C., Ransolin, N,. & Disconzi, C. G.. (2021). Slack in Construction – Part 2: Practical Applications. In Proceedings of the 29th Annual Conference of the International Group for Lean Construction (pp. 197-206). IGLC. https://doi.org/10.24928/2021/0178