How to Calculate the Number of Observations Required for Time Study

Time study is one of the most important activities in industrial engineering. It is the foundation for manpower calculation, capacity planning, line balancing, costing, incentive calculation, and productivity improvement.

But in many factories, time study is misunderstood.

Some people take 3 readings and decide the cycle time.
Some people take 5 readings and calculate manpower.
Some take one “average-looking” reading and use it for planning.
Some take more readings only when someone questions the result.

This creates a serious problem.

If the observed time is not reliable, the standard time will not be reliable. If the standard time is not reliable, manpower calculation will also become wrong.

In simple words:

Cycle time wrong → Standard time wrong → Manpower wrong → Capacity wrong → Costing wrong

That is why every industrial engineer should understand one important question:

How many observations are required in a time study?

This article explains the concept in a simple and practical way.

Why One Observation Is Not Enough in Time Study

Let us say we observe one operator doing an operation and the stopwatch reading is:

18 seconds

Can we immediately say the operation time is 18 seconds?

No.

Because if we observe the same operation again, the next readings may be:

18 seconds, 19 seconds, 20 seconds, 18 seconds, 21 seconds

This happens because every operation has natural variation.

The variation may come from many reasons:

Operator movement variation
Part location variation
Material handling variation
Machine response variation
Tool condition
Part orientation
Fatigue
Minor delays
Measurement error
Difference in work rhythm

So time study is not just about holding a stopwatch. It is about collecting enough observations to find a reliable average.

What Are We Trying to Find in Time Study?

In time study, we are trying to find the representative average time for each element of work.

For example, an operation may be divided into elements like:

Pick the part
Load the part
Press the switch
Wait for machine cycle
Unload the part
Keep the part in tray

Each element may have a different level of variation.

Machine time may be very stable. Manual handling time may vary more. Searching, positioning, checking, walking, or aligning may vary even more.

So the number of observations required may not be the same for every element.

This is the key idea:

The more the variation, the more observations are required.

What Decides the Number of Observations?

The number of observations required in a time study mainly depends on three factors:

1. Variation in readings
2. Confidence level
3. Accuracy or margin of error

These three points are connected, but they are not the same. Let us understand them clearly.

1. Variation in Readings

Variation means how much the observed times are changing from cycle to cycle.

If the readings are very close to each other, fewer observations may be enough.

Example:

18, 18, 19, 18, 19

This is low variation. The operation looks stable.

But if the readings are like this:

12, 18, 25, 14, 30

This is high variation. The operation is not stable, or the method may not be consistent. In such cases, more observations are required.

In simple words:

Low variation = fewer observations may be enough.
High variation = more observations are required.

2. Confidence Level

Confidence level means how sure we want to be about our result.

A simple way to understand this is:

If we repeat the same time study 100 times under similar conditions, around 95 studies should give a result within the accepted error range (margin or error or accuracy – explained in next session).

In time study, we take some observations and calculate an average. But that average is only an estimate. It may not be exactly equal to the true average time of the operation.

For example, suppose we observe an operation 10 times and get an average of:

20 seconds

Can we say the true average is exactly 20 seconds?

Not exactly.

If we take more observations, the average may become 19.8 seconds or 20.3 seconds. So the observed average is only an estimate of the true average.