Time Study Sample Size Calculator

Work out time study sample size instantly with clear inputs, formula shown and shareable results.

About Time Study Sample Size Calculator

For a continuous variable such as cycle time, sample size is n = (z x coefficient of variation / relative accuracy)^2. Variability drives everything: a task with a 10% coefficient of variation needs about 16 cycles for 5% accuracy at 95% confidence, while one at 30% needs around 139.

Formula

Time study sample size

n = (z x s / (accuracy x mean))^2

Worked example

With the default values below, Time Study Sample Size Calculator returns Cycles To Observe of 31. Change any input above and every figure here is recalculated in your browser.

Inputs used

Mean Cycle Time From Pilotminimum 0.001 minutes: 2.5 minutes
Standard Deviationminimum 0 minutes: 0.35 minutes
Desired Relative Accuracyaccepts 0.1–50 %: 5 %
Confidence Levelone of 90%, 95%, 99%: 95%

Results

Cycles To Observe: 31
Coefficient Of Variation: 14.00%
Z Value Used: 1.960
Margin Of Error: 0.1250

Step by step

1Values used
2Time study sample size
3Cycles To Observe= 31
4Coefficient Of Variation= 14.00
5Z Value Used= 1.960
6Margin Of Error= 0.1250

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why does high variability need so many cycles?

Sample size grows with the square of the coefficient of variation. Rather than observing more, it is usually better to find and remove the source of variation — inconsistent method, material or machine condition.

Should outlier cycles be excluded?

Only if the cause is identified and genuinely non-recurring, such as a tool break. Excluding slow cycles because they are inconvenient produces a standard nobody can achieve.