Sequential chain
T_seq = Σ tᵢ — the foundation of every other calculation.
The simplest formula, and the foundation of all the others.
The formula
Tseq=∑i ∈ Sti
Cseq=∑i ∈ Sci
(1)
Where S is a chain of steps running one after another, with no branching. Time and cost follow the same rule: add them up.
Why it holds
A step only begins once the previous one has finished, so the elapsed time of the whole chain is exactly the sum of the individual step times. No resource is shared, nothing overlaps.
Where waiting time goes
If there is a significant wait between two steps (waiting for a customer to reply, waiting for goods to arrive), create a separate step for it with a large time and zero cost. Folding the waiting time into the work step hides an important truth: the process is slow because of waiting, not because of working.
If there is a significant wait between two steps (waiting for a customer to reply, waiting for goods to arrive), create a separate step for it with a large time and zero cost. Folding the waiting time into the work step hides an important truth: the process is slow because of waiting, not because of working.
Example
Three steps in sequence
| Step | Time | Cost |
|---|---|---|
| Receive the file | 10 minutes | 30.000 |
| Assessment | 25 minutes | 120.000 |
| Approval | 15 minutes | 80.000 |
Tseq=10+25+15=50 minutes
Cseq=30.000+120.000+80.000=230.000
Why there is no dedicated formula
A sequential chain is not handled separately — it is the special case of the general formula E[T] = ∑ nv tv where every nv = 1. See Composing nested blocks.
Anything unclear or missing?