Shortest Processing Time

Shortest Processing Time (SPT) is a sequencing rule in Intro to Industrial Engineering that schedules the job with the shortest processing time first. It is used to reduce average flow time and keep work moving through a job shop faster.

What is Shortest Processing Time?

Shortest Processing Time, or SPT, is a job sequencing rule in Intro to Industrial Engineering that puts the shortest job at the front of the line. If two or more jobs are waiting for the same machine or workstation, you choose the one that will take the least time to finish first.

In a job shop, this matters because each job may have a different processing time and a different route through the system. SPT is one of the simplest priority rules, but it is not random. It uses a clear rule: the quicker a job can be completed, the earlier it gets processed.

The big effect of SPT is on flow time, which is the total time a job spends in the system from start to finish. When short jobs are completed early, the average flow time for the whole set of jobs usually drops. That also tends to reduce work in progress, because fewer jobs sit waiting while long jobs block the queue.

A small example makes the logic easier to see. If a machine has jobs that take 2 hours, 5 hours, and 9 hours, SPT schedules them in that order. The 2 hour job finishes quickly, the 5 hour job does not wait as long, and the average completion time across all three jobs is lower than if you started with the 9 hour job.

The tradeoff is fairness. Long jobs can wait a long time under SPT, so if a shop only uses this rule, some bigger jobs may get pushed back again and again. That is why industrial engineers often treat SPT as one rule in a larger scheduling plan, not the only rule in the system. It works best when processing times are known ahead of time and the main goal is speed through the system, not equal waiting time for every job.

Why Shortest Processing Time matters in Intro to Industrial Engineering

SPT shows up any time you need to decide which job should go first in a job shop. It connects directly to job shop scheduling, because the order of jobs changes how long work waits, how much inventory piles up, and how smoothly a machine stays busy.

This rule is a good example of the kind of decision-making industrial engineering cares about. You are not just finishing tasks, you are choosing an order that improves a system outcome. With SPT, the outcome is usually lower average completion time, shorter cycle time, and less congestion at busy stations.

It also helps you see the tradeoff between efficiency and fairness. A schedule can look great on average while still being rough on one or two long jobs. That is a common theme in industrial engineering, where the best rule depends on what you are trying to optimize.

SPT is especially useful in course problems because it is easy to calculate and easy to compare against other sequencing rules. If a homework or quiz problem gives you several jobs with known times, SPT is often the first rule you test when the question asks about flow time, waiting time, or queue order.

How Shortest Processing Time connects across the course

Job Shop Scheduling

SPT is one sequencing rule used inside job shop scheduling. In a job shop, different jobs often have different processing times and different routes, so the order you pick can change how long the system stays crowded. SPT is one of the simplest ways to choose a queue order when a machine has several waiting jobs.

Priority Rules

SPT is a priority rule because it assigns a job to the front of the line based on a rule, not on guesswork. Other priority rules may focus on due dates, urgency, or ratios, so comparing them helps you see what each rule optimizes. SPT is usually about speed through the system, not deadline protection.

Cycle Time

SPT often lowers cycle time because jobs spend less time waiting before processing. In a scheduling problem, cycle time is one of the clearest outcomes to watch when you change the job order. If short jobs are cleared first, the system can move faster overall even if one long job waits longer.

Bottleneck Analysis

SPT can reduce pressure near a bottleneck by clearing short jobs faster and keeping the queue moving. That does not remove the bottleneck itself, but it can make the line less clogged. When you analyze a busy workstation, SPT is one rule you can test to see whether waiting time drops.

Is Shortest Processing Time on the Intro to Industrial Engineering exam?

A scheduling question may give you several jobs with different processing times and ask for the order under SPT. You would sort the jobs from shortest to longest processing time, then use that sequence to compare completion times or average flow time. If the problem asks which rule reduces average completion time, SPT is usually the right choice.

You may also have to explain the tradeoff. A short written response can ask why one long job finishes late even though the schedule is efficient overall. In that case, you should point out that SPT favors quick jobs and can increase waiting time for longer jobs. In problem sets, labs, or class discussions, this often comes up when you compare SPT with a due date based rule or look at how the queue changes at a machine center.

Shortest Processing Time vs Critical Ratio

SPT and Critical Ratio are both scheduling rules, but they sort jobs using different logic. SPT looks only at processing time and puts the shortest job first. Critical Ratio combines time left until due date with processing time, so it focuses more on urgency than on speed alone.

Key things to remember about Shortest Processing Time

• Shortest Processing Time schedules the shortest job first in a queue or machine center.

• SPT is used to reduce average flow time and keep work moving through a job shop more quickly.

• The rule works best when processing times are known ahead of time and jobs are easy to compare.

• SPT can improve efficiency, but long jobs may wait longer and feel pushed to the back.

• In Intro to Industrial Engineering, SPT is often compared with other priority rules to see which one fits the goal of the system.

Frequently asked questions about Shortest Processing Time

What is Shortest Processing Time in Intro to Industrial Engineering?

Shortest Processing Time is a sequencing rule that puts the job with the smallest processing time first. In Intro to Industrial Engineering, you use it to reduce average flow time and keep a job shop moving efficiently.

Why does SPT reduce average flow time?

SPT gets short jobs finished quickly, so fewer jobs sit in the queue for long periods. That lowers the total waiting time across the set of jobs, which is why the average completion time usually drops.

What is the downside of Shortest Processing Time?

The main downside is that long jobs can wait a long time. SPT is efficient, but it can create fairness problems if the same long jobs keep getting pushed back.

How do you use SPT in a scheduling problem?

List the jobs by processing time from smallest to largest, then follow that order when you calculate completion times or flow time. If the question asks you to compare rules, SPT is the rule that focuses on the shortest job first, not the most urgent job first.