Introduction to Dependencies

Dependencies inform how multiple tasks within a project must be sequenced and scheduled.

Establishing dependencies between multiple tasks in a project plan is an important part of turning a task list into a timeline. Dependencies describe relationships between tasks and inform project managers and team members how tasks need to be sequenced. Projects of every size have dependent tasks and a specific sequence in which some tasks need to be completed.

Think about making an omelet as a series of three steps: (1) crack the eggs, (2) cook the omelet, (3) plate the omelet. There is only one order in which these tasks can be completed – you can’t cook your eggs before you’ve opened them, and you can’t plate your omelet before it's cooked. Each of these tasks is dependent on its predecessor.

Not every task in a project has a dependency. For example, you need to get out a knife and fork to eat your omelet, but it doesn’t matter if you do this before, during, or after the eggs are cooking.

Understanding Dependencies

Dependencies between tasks are an important component of project management. Together with constraints, they inform how multiple tasks within a project must be sequenced and scheduled. Dependencies dictate the nature of the relationship between two tasks.

Each dependency involves a successortask and a predecessor task. A successor task is reliant on a predecessor task in some way.

For example, if you’re building a house, you can’t install the windows until you’ve put up the framing. “Window installation” is a task that must succeed the completion of “framing”, its predecessor. Or if you’re launching a new product, you can’t put together an ad campaign until you’ve identified your target demographic; the former must succeed the latter.

Almost every project involves dependencies, but these dependencies come in different forms. The beginning of some tasks are dependent on other tasks beginning, while others are dependent on other tasks finishing. Sometimes, the ending of a task is dependent on another task’s beginning or completion.

Read on to learn more about different types of dependencies, or skip ahead and learn how to create and manage task dependencies in a Gantt view.

Types of Dependencies

Spreadsheet.com supports four different types of dependencies to reflect the different relationships between your project tasks:

Type

Abbreviation

Description

Finish-to-Start

FS

A task must finish before its successor task can start

Start-to-Start

SS

A task must start before its successor task can start

Start-to-Finish

SF

A task must start before its successor task can finish

Finish-to-Finish

FF

A task must finish before its successor task can finish

Let’s take a closer look at each type of dependency:

finish-to-start.png

When two tasks have a Finish-to-Start (FS) dependency, the predecessor task (Task 1) must finish before the successor task (Task 2) can begin. This is the most common type of dependency found in project management. Because of the nature of the dependency, these tasks may not overlap at all and must be completed sequentially.

start-to-start.png

When two tasks have a Start-to-Start (SS) dependency, the predecessor task (Task 1) must start before the successor task (Task 2) can start. With a Start-to-Start dependency, the end dates of both the predecessor and successor tasks are irrelevant to their relationship. These tasks may overlap partially, entirely, or not at all, but the predecessor task must begin before the successor task does.

start-to-finish.png

When two tasks have a Start-to-Finish (SF) dependency, the predecessor task (Task 1) must start before the successor task (Task 2) can finish. These tasks may overlap partially, entirely, or not at all, and the predecessor task may begin after the successor task begins.

finish-to-finish.png

When two tasks have a Finish-to-Finish (FF) dependency, the predecessor task (Task 1) must finish before the successor task (Task 2) can finish. With a Finish-to-Finish dependency, the start dates of both the predecessor and successor tasks are irrelevant to their relationship. These tasks may overlap partially, entirely, or not at all, and the predecessor task may begin after the successor task begins.

Read on to learn more about constraints, another key component of scheduling and sequencing project tasks. Or, read on to learn more about creating and managing task dependencies in Spreadsheet.com.

Lag Time

Lag time describes the duration of a dependency between two tasks. In Spreadsheet.com, lag times are expressed in days or hours and can be negative, zero, or positive values. The most common type of scheduling is a dependency of zero days, also known as “As Soon as Possible”, or ASAP. This means that a successor task can begin as soon as its dependency criteria is met.

Let’s consider our previous omelet example again. Once the omelet is done cooking, there’s no need to wait to put it in the pan – it can (and should) happen as soon as possible. In project management terms, these tasks have a Finish-to-Start dependency lag of 0 days.

zero-lag.png

Or, consider a road repaving project. Once new asphalt is poured, it takes about 72 hours for it to be dry enough for people to drive on. If “pour asphalt” is the predecessor task and “reopen road” is the dependent task, there would be a Finish-to-Start dependency with a lag of +3 days.

positive-lag.png

Lag can be applied to all types of dependencies, not just Finish-to-Start. When applied to Start-to-Start dependencies, for example, lag can indicate that two tasks may run concurrently, but that the predecessor task needs a “head start” before the successor task can begin. When framing a house over ten days, maybe we can begin installing some windows three days before the framing ends, giving us a lag of -3 days.

negative-lag.png

Learn more about working with lag in our article on creating and managing task dependencies in Spreadsheet.com. Or, read on to learn about more project management features like constraints and the critical path.