Input/Output Control

Input/Output Control in Detailed Scheduling – A Comprehensive CPIM Guide

Introduction to Input/Output Control

Input/Output (I/O) Control is a critical technique within detailed scheduling and planning that monitors and manages the flow of work into and out of a work center. It is one of the most practical and testable topics in the CPIM (Certified in Planning and Inventory Management) exam, particularly within the Detailed Scheduling and Planning module. Understanding I/O Control is essential for managing shop floor operations, controlling work-in-process (WIP) inventory, and maintaining realistic lead times.

Why is Input/Output Control Important?

Input/Output Control is important for several key reasons:

1. Controls Work-in-Process (WIP) Inventory: By monitoring the balance between work arriving at and leaving a work center, I/O Control helps keep WIP at manageable levels. Excessive WIP leads to longer lead times, higher carrying costs, congestion on the shop floor, and reduced visibility of priorities.

2. Maintains Realistic Lead Times: If input consistently exceeds output, queues grow and actual lead times stretch beyond planned lead times. I/O Control provides early warning of this imbalance so corrective action can be taken before the situation deteriorates.

3. Identifies Capacity Problems Early: The I/O report highlights whether a work center is falling behind its planned output, enabling management to investigate root causes such as machine breakdowns, absenteeism, tooling issues, or material shortages.

4. Supports Capacity Management: It is a key execution tool in the capacity management hierarchy that connects capacity requirements planning (CRP) to the actual shop floor performance.

5. Validates Planning Assumptions: By comparing planned versus actual input and output, I/O Control provides feedback that validates or challenges the assumptions used in planning.

What is Input/Output Control?

Input/Output Control is a technique for monitoring and managing the actual flow of work (measured in standard hours, units, or other capacity measures) entering and leaving a work center, and comparing these actual values to planned values. The results are displayed in an Input/Output Report (also called an I/O Report or I/O Control Report).

The I/O Report typically contains the following elements:

- Planned Input: The amount of work scheduled to arrive at the work center during a given period.
- Actual Input: The amount of work that actually arrived at the work center during that period.
- Cumulative Input Deviation: The running total of the difference between planned and actual input.
- Planned Output: The amount of work scheduled to be completed at the work center during a given period.
- Actual Output: The amount of work actually completed during that period.
- Cumulative Output Deviation: The running total of the difference between planned and actual output.
- Planned Backlog (Queue): The expected queue or backlog of work waiting at the work center.
- Actual Backlog (Queue): The actual queue of work waiting at the work center.

The fundamental relationship is:

Ending Backlog = Beginning Backlog + Input – Output

This simple equation is the foundation for understanding I/O Control and is frequently tested on the exam.

How Does Input/Output Control Work?

Here is a step-by-step explanation of how I/O Control works in practice:

Step 1: Establish Planned Input and Output
Using the capacity requirements plan (CRP) and the released and planned shop orders, the planner determines the planned input and planned output for each work center over a series of time periods (typically weeks).

Step 2: Set the Initial Backlog
The starting backlog (or queue) is determined from the current status of work waiting at the work center. This is the beginning WIP at the work center.

Step 3: Record Actual Input and Output
As work flows through the shop floor, the actual input (work arriving) and actual output (work completed) are tracked and recorded each period.

Step 4: Calculate Deviations
The differences between planned and actual values for both input and output are calculated each period, and cumulative deviations are maintained:

Cumulative Input Deviation = Σ (Actual Input – Planned Input)
Cumulative Output Deviation = Σ (Actual Output – Planned Output)

Step 5: Calculate Actual Backlog
Using the fundamental equation:
Actual Backlog (end of period) = Actual Backlog (start of period) + Actual Input – Actual Output

Step 6: Analyze and Take Corrective Action
Management reviews the I/O Report and takes corrective actions based on the patterns observed.

Key Scenarios and Their Implications:

- If Actual Input > Actual Output: The backlog (queue) is growing. Lead times will increase. Action may be needed to either reduce input (by rescheduling orders) or increase output (by adding overtime, extra shifts, or subcontracting).

- If Actual Input < Actual Output: The backlog is shrinking. If this continues, the work center may run out of work (starve), leading to idle capacity. Action may be needed to release more work or reduce capacity.

- If Actual Input ≈ Actual Output: The backlog is stable. However, it is important to check whether the backlog is at the desired level. A stable but excessively high backlog still means lead times are too long.

- If Actual Output < Planned Output (Cumulative Output Deviation is negative): The work center is falling behind schedule. Root causes should be investigated – machine problems, labor shortages, quality issues, material shortages, etc.

- If Actual Input > Planned Input (Cumulative Input Deviation is positive): More work is arriving than expected. This could be due to upstream work centers completing work faster than planned, or unplanned orders being released. This will increase the backlog unless output also increases.

Controlling Input vs. Controlling Output:

A key concept in I/O Control is that input is generally easier to control than output. Input can be controlled by managing the release of orders to the shop floor, adjusting the master production schedule, or rescheduling orders. Output, on the other hand, is constrained by available capacity (equipment, labor, tooling) and is harder to change quickly.

This leads to the important principle: To control WIP and lead times, control the input. This is sometimes called the funnel analogy – if you pour too much water into a funnel (input), it overflows (queue builds up). The solution is to regulate the flow into the funnel rather than trying to force more through the spout.

Tolerance Limits:
In practice, organizations set tolerance limits for cumulative deviations. When the cumulative deviation exceeds the tolerance, it triggers a management review and corrective action. For example, if the cumulative output deviation exceeds -20 standard hours, management may authorize overtime.

Relationship to Other Concepts:

- Capacity Requirements Planning (CRP): I/O Control is the execution monitoring tool that validates CRP plans. CRP generates the planned input and output values that I/O Control tracks against actual performance.

- Queue Management: I/O Control directly affects queue levels, which are a major component of manufacturing lead time. Reducing queues through better I/O Control directly reduces lead times.

- Production Activity Control (PAC): I/O Control is a key element of PAC, providing the capacity management component alongside priority management (dispatching).

- Theory of Constraints (TOC): At bottleneck work centers, I/O Control is especially critical. The output of a bottleneck determines the throughput of the entire system.

Worked Example:

Consider a work center with the following data (in standard hours):

Beginning Backlog: 40 hours

Period 1: Planned Input = 30, Actual Input = 35, Planned Output = 30, Actual Output = 28
Period 2: Planned Input = 30, Actual Input = 25, Planned Output = 30, Actual Output = 32

Period 1 Calculations:
- Cumulative Input Deviation = 35 – 30 = +5
- Cumulative Output Deviation = 28 – 30 = –2
- Actual Backlog = 40 + 35 – 28 = 47 hours

Period 2 Calculations:
- Cumulative Input Deviation = +5 + (25 – 30) = +5 – 5 = 0
- Cumulative Output Deviation = –2 + (32 – 30) = –2 + 2 = 0
- Actual Backlog = 47 + 25 – 32 = 40 hours

Notice how the backlog returned to its original level because over two periods, total actual input (60) equaled total actual output (60).

Exam Tips: Answering Questions on Input/Output Control

1. Master the Core Formula: Always remember: Ending Backlog = Beginning Backlog + Input – Output. This formula is the basis for most calculation questions on I/O Control. Write it down immediately when you see an I/O question.

2. Watch for Cumulative Deviations: Many questions ask about cumulative (not just periodic) deviations. Keep a running total. A common mistake is to calculate only the deviation for the current period and forget the previous periods' cumulative total.

3. Understand the Cause and Effect: Exam questions often present a scenario and ask what is happening or what action to take. If input exceeds output, the queue grows and lead times increase. If output exceeds input, the queue shrinks and the work center may eventually starve.

4. Know That Input Is Easier to Control: If a question asks about the best way to reduce WIP or queues, controlling (reducing) input to the work center is generally the preferred answer. Increasing output is possible but typically harder and more costly.

5. Distinguish Between Planned and Actual: Questions may try to confuse you by mixing planned and actual values. Read carefully. The planned backlog uses planned input and output; the actual backlog uses actual input and output.

6. Remember Tolerance Limits: When a question mentions tolerance limits, it is asking about the threshold at which management action should be triggered. If the cumulative deviation is within tolerance, no immediate action is required.

7. Connect I/O Control to the Bigger Picture: Some conceptual questions ask about the role of I/O Control in the overall planning and control system. Remember: I/O Control is a capacity control technique used during execution. It is part of Production Activity Control (PAC) and validates the Capacity Requirements Plan (CRP).

8. Look at Both Input and Output Sides: When analyzing an I/O report in an exam question, consider both the input side and the output side. A growing backlog could be caused by too much input, too little output, or both. The corrective action depends on the root cause.

9. Negative Cumulative Output Deviation Signals Trouble: If the exam describes a consistently negative cumulative output deviation, this means the work center is chronically underperforming. Look for answers related to investigating root causes, adding capacity, or adjusting the schedule.

10. Be Careful with Units: I/O Control can be measured in standard hours, units, or other measures. Make sure you are using consistent units throughout your calculations.

11. Practice with Sample I/O Tables: The best preparation for I/O Control exam questions is to practice filling out and interpreting I/O reports. Work through multiple examples until you can quickly calculate backlogs and cumulative deviations.

12. Remember the Funnel Analogy: If you encounter a question about managing WIP, think of the funnel. The key insight is that you manage queues by managing input, not just by trying to increase throughput. This is a commonly tested conceptual point.

Summary

Input/Output Control is a foundational technique in detailed scheduling that ensures work centers operate effectively by monitoring the balance between incoming and outgoing work. It prevents excessive WIP buildup, keeps lead times under control, and provides early warning of capacity issues. For the CPIM exam, focus on the core backlog formula, understanding the implications of input/output imbalances, knowing that controlling input is the primary lever for managing WIP, and being able to calculate and interpret cumulative deviations from an I/O report. Mastering these concepts will equip you to confidently answer both calculation and conceptual questions on this topic.