Critical ratio is a job shop scheduling rule that compares the time remaining until a due date to the processing time needed for a job. In Intro to Industrial Engineering, you use it to spot which jobs are most at risk of being late.
Critical ratio is a priority rule in Intro to Industrial Engineering that tells you how urgently a job should be processed. You calculate it with CR = time remaining until due date divided by the job’s processing time.
If the ratio is below 1, the job does not have enough time left to finish on schedule, so it is behind and should move up the queue. If the ratio is exactly 1, the job is right on the edge. If it is above 1, the job has some cushion, so it can usually wait behind more urgent work.
The idea is simple, but it fits job shop scheduling very well. A job shop handles many different jobs, often with different routings and due dates, so managers need a quick way to rank work instead of guessing based only on due date or only on processing time. Critical ratio combines both pieces of information in one number.
The rule is dynamic, not fixed. As time passes, the remaining time changes, so a job’s critical ratio can drop even if nothing about the job itself changes. That means you can recalculate it during the day to see whether a job has become more urgent after a delay, machine breakdown, or long setup time.
A small example makes the pattern clearer. Suppose a job has 6 hours left before its due date and needs 4 hours of processing. Its critical ratio is 6/4 = 1.5, so it is ahead of schedule. If another job has 3 hours left and still needs 5 hours of work, its critical ratio is 3/5 = 0.6, which signals a late risk. In a scheduling exercise, that second job would usually get priority.
One common mistake is mixing up critical ratio with shortest processing time. They are not the same. Shortest processing time only looks at how long the job takes, while critical ratio compares that time to the deadline clock too. That makes critical ratio better when due dates matter and when you want to reduce lateness, not just keep the machine busy.
Critical ratio gives you a fast way to make sequencing decisions when there are too many jobs to schedule by instinct. In a job shop, you may have custom orders, different due dates, and machines that do not all handle the same work. A rule like critical ratio helps you decide what goes next without rebuilding the whole schedule every time one job changes.
It also connects directly to the course’s bigger themes of optimization and process improvement. You are not just counting hours, you are balancing urgency, machine time, and the risk of tardiness. That is the kind of thinking industrial engineers use when they want to improve throughput while still meeting customer deadlines.
This term also shows up in tradeoffs. A job with a low critical ratio might need immediate attention even if it is not the shortest job in the system. That can change how you think about bottlenecks, resource allocation, and the cost of lateness. In a class problem, the right answer often depends on whether you are trying to minimize late jobs, reduce lateness, or keep a schedule realistic under changing conditions.
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Visual cheatsheet
view galleryJob Shop Scheduling
Critical ratio is a sequencing rule used inside job shop scheduling. In a job shop, each job may follow a different route and have a different due date, so you need a rule that ranks jobs quickly. Critical ratio works well there because it compares urgency against remaining work instead of using only one factor.
Due Date
The due date is the deadline that goes into the critical ratio calculation. If the due date gets closer, the remaining time shrinks and the ratio drops. That makes due date one of the main inputs that changes a job’s priority over time.
Processing Time
Processing time is the other half of the formula. A long processing time makes the critical ratio smaller, which signals that a job needs more lead time to finish on time. In class problems, students often forget that the job can be urgent either because the deadline is near or because the job itself is long.
Flow Shop Scheduling
Flow shop scheduling is a useful contrast because jobs usually move through the same sequence of machines. Critical ratio is especially tied to job shop settings, where routes are less predictable. Comparing the two helps you see why different scheduling rules fit different production layouts.
A scheduling problem or quiz question will usually give you the time left, the processing time, and a list of jobs. Your job is to compute each critical ratio, compare the values, and rank the jobs from most urgent to least urgent. Watch for wording like "which job should be processed first" or "which job is at risk of being late."
If the ratio is less than 1, that is your warning sign that the job is behind schedule. If two jobs are both under 1, the smaller ratio is usually the more urgent one. Many mistakes come from using due date alone, so always check whether the problem expects the ratio, not just the earliest deadline.
Critical ratio compares time left until the due date with the time needed to finish the job.
A value below 1 means the job is in trouble and needs priority.
A value above 1 means the job has some slack and can usually wait longer.
The rule is especially useful in job shop scheduling, where jobs have different routes and deadlines.
Unlike shortest processing time, critical ratio looks at both urgency and work remaining.
Critical ratio is a scheduling priority rule that divides the time remaining until a job’s due date by its processing time. It tells you which jobs are most urgent in a job shop. The smaller the ratio, the more at risk the job is for being late.
Use CR = time remaining / processing time. First find how much time is left before the due date, then divide by the total time the job still needs. If the result is below 1, the job is behind schedule.
No. Due date priority only looks at the deadline itself, while critical ratio also includes how long the job still takes. That means a job with a later due date can still be more urgent if its processing time is long enough.
A smaller ratio means the remaining time is short compared with the work left to do. That job has less slack, so delaying it makes lateness more likely. In scheduling problems, that is usually the job you move to the front of the line.