Ergo Tip of the Month

April 2006

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Quantifying Physical Demands:
Data Collection Tips for Measuring Forces

using a biomechanical model or measuring muscular effort with electromyography. However, it is not only important to have a strong methodology for predicting the physical capacity of a worker, but also to accurately quantify the demands of the job. In particular, when determining what the force requirements are for a task, one will usually employ the use of a weigh scale or force gauge. Weigh scales can be used to determine the mass of an object if it is to be lifted/lowered, whereas a force gauge can not only quantify a lift force, but can also be used to determine push and pull

forces too. It is important to ensure that whenever quantifying the force requirements of a task, that proper data collection procedures are followed. For example, in a manufacturing environment, if a worker is required to snap together two components, such as trim on a vehicle, the casing on a lap top computer, or 2 ends of an electrical connector, the Ergonomist will want to ensure the force required to perform these activities does not exceed the capabilities of the worker.

Tips for Successful Force Measurement:

1. Use a Force Gauge:
   Ensure you are familiar with the use of the force gauge.
   
   Avoid common pitfalls of force gauge use i.e. applying force too rapidly, applying force on an angle, etc.
   


2. Ensure Appropriate Sample Size:
   A minimum sample of 6 parts/component is recommended, 30 is ideal. The actual sample size depends on the coefficient of variation (CV) of your sample.
   


3. Use new or virgin parts/components:
   If you are measuring the lift/lower force of an object, it is sufficient to use the same part and measure it repeatedly. However, if you are measuring the force of 2 objects snapping or mating together, it is important to use new parts for each trial.
   


4. Record your forces and do your statistics:
   You will need to calculate the mean (or average) of your sample, which is the total of the sample divided by the sample size. Next, you can calculate your Standard Deviation (SD), which is the square root of the variance.
   


5. Compare the average force to your measure of physical capability:
   If the average force of your sample is below the force you have deemed acceptable for the task, then no changes are needed. However, if the average force calculated for your trials exceeds the physical capabilities of the worker, then a change to the task requirements is recommended.
   

What if the parts you need to measure cannot be accessed with a force gauge?

Stay tuned for an upcoming Sandalwood "Ergo Tip of the Month" that will address this issue...

To Summarize:

When you need to determine the force requirements for a task, first ask yourself whether you can access the part/component with a force gauge or not, and then follow through with the appropriate protocol to ensure a successful outcome.

References:
Gescheider,G.A. (1985) Psychophysics: Method, Theory, and Application. Lawrence Erlbaum Associates, Inc., Hillsdale, New Jersey.

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