Fil Salustri's Design Site

Site Tools


Human Factor Demands

Before determining the human factor capabilities you wish to design for, you must understand the human factor demands put upon users by the reference design.

What are Human Factor Demands?

In engineering design, a human factor (HF) is a quantifiable characteristic of a human that will impact how the human and a designed artifact interact.

A human factor demand is an expectation embedded in a product's design of what a user can do. Most poorly designed products are that way because they place unreasonable demands on users.

Since no two humans are the same, HFs are typically represented as ranges of values. The minima and maxima of these ranges mark the extrema outside of which harm or inefficiency result during human-artifact interactions. These ranges are in turn defined with respect to human capabilities.

HF demands are the “worst case” of the extrema that mark the range of an HF.

In principle, the wider the range of HFs for a particular design, the more potential users there are for the resulting product; however, this typically results in more complex products, which in turn increases costs and decreases durability and reliability. On the other hand, a very small range of HFs can allow the design of cost effective, efficient, and reliable products; however, this reduces the range of users who can use the product by typically excluding and marginalizing those users who are “far from average”.

One must therefore seek to balance the ranges of HFs so as to maximize the range of potential users while also making a product that is reasonably reliable, cost-effective, and efficient.

It doesn't matter if you create a truly “universal” design if it costs so much that only a handful of people can afford it. Nonetheless, there is a growing expectation - even with corporations - that designing for the broadest possible range of user (i.e., diversity) is essential for both corporate social responsibility and financial success; for instance, consider this case and this case. The challenge of design is to find that balance between all factors - both technical and human - to create the best possible product for a given situation.

Thus, one must study human capability to set appropriate ranges of design HFs, to in turn design a usable product.

Furthermore, not all possible HFs are relevant in a specific project. Thus, one must prioritize those HFs that are most relevant to the scope of a given design brief.

Example 1: Night vision devices are predicated on users having some minimal amount of vision; users who are completely blind are excluded by definition from this class of product. Sometimes, this kind of exclusion is unavoidable, but the rationale for such exclusions should be clear, well-documented, and not founded on biases or discriminatory principles.

Example 2: Both surgical clamps and locking pliers provide the same basic function, but you wouldn't want your surgeon operating on you with pliers. The difference is the scope and context in which the required function is to be implemented and the capabilities of the users in each case.

We refer to the overall scope of all HF ranges as the human factors envelope for a product.


Figure 1: A typical kitchen blender.

Consider how one might use the blender in Figure 1.

Did you remember to consider situations where a user would have to walk a short distance while holding the blender's container, and stand while dispensing its content?

How many people do you think this simple and entirely reasonable expectation might exclude from a general population? Think about that before viewing some possible results below.

Click here to see the exclusion data.

How do we establish human factor demands?

In this introductory design course, you are not expected to display expert-level skill in determining HFs. However, you are expected to:

  • prioritize HFs from a list of common ones,
  • use existing scales to quantify (at least on an ordinal scale) the ranges of those HFs, and
  • justify your decisions with respect to the design brief and ergonomics knowledge available in this course.

The HFs you should consider are provided in the Human Factors and Personas chart, an example of which is given below. Which HFs you actually use in your project will depend on many factors. The overall scope of all these HFs and their ranges defines the HF envelope of a product.

If you cannot find specific sources for the specific HFs you have identified, you can use the following generic scale. This scale is meant to identify the percentage of your targeted user population that you exclude from being able to use a design. The scale is based on a normal distribution, which is a reasonable “best-guess” for many sufficiently large populations.

Figure 2: Percentages of a normal
distribution (source).

The five acceptable levels for this scale are:

  • 0.1% : Only excluding the most extreme cases.
  • 2.2% : Excluding a significant but not necessarily problematic subpopulation.
  • 15.8% : Excluding a significant subpopulation that is likely problematic.
  • 84.2% : Excluding most of the target population - see you at bankruptcy court!
  • 97.8% : Seriously?

It's important to remember, however, that there may be excellent reasons for excluding significant number of potential users. It's up to you to justify that.

This notwithstanding, strong preference is given to using specific scales developed by scientists and ergonomists. We have developed information to assist you in determining some HF ranges for both demands and capabilities1).

Teams are expected to use the information given below wherever possible. If information is missing, they may use the default assessment system given above.

Below is a general procedure you must follow.

Step 1: Make a copy of the HF&P chart

Each team needs a single copy of the Human Factors and Persona Chart2).

This chart must be included in your design reports, and must be completed collaboratively by all team members.

Step 2: Analyze the usage scenarios

Each team member analyzes their SUC in conjunction with the team's usage scenario. That is, you consider the SUC as a “real-life” situation in which some user will try to follow the usage scenario.

For each HF (per the Human Factors and Persona Chart), look for a US step that you believe will embody the “worst case” for that HF - i.e., the step that you believe will put the most severe demand on users. For instance:

  • a product is too heavy if users who are expected to lift it either cannot lift it or are harmed by having to lift it;
  • labels are too small if users who must read the labels cannot do so easily;
  • user interfaces are too complicated if users who are expected to interact with it can't figure out how.

Given what you know about the reference design, estimate the value of each HF.

Make a list of all the HFs you've found, the value of each HF, and a rationale for having assigned that value to that HF.

Step 3: Create a prioritized list of HF demands

Prioritize the list of HF demands from Step 2, with respect to:

  • severity: How severe you believe the demand is that the HF places on human users?
  • likelihood: How likely is it that users will have to meet that demand?
  • frequency: How often will users be expected to meet that demand?

You may use a simple 3-point ordinal scale for each item above, defined as follows:

not severe/likely/frequent at all.
moderately severe/likely/frequent.
very severe/likely/frequent. Then you add the severity, likelihood, and frequency scores together to get an overall measure of the impact of the HF demands. You can now order that list from most to least impactful.

Step 4: Identify the worst cases over all SUCs

In your teams, share you ordered lists from Step 3, merge them together, and resort the overall list by impact. Furthermore, remove any duplicates that might appear in the overall list.

Search the list for the worst-of-the-worst cases for each HF. These will be the overall worst cases, one for each HF, for which you will conduct detailed analyses, per the next step.

In real life, every HF demand on the list would have to be analyzed in detail, but in this academic setting we only expect each team member to demonstrate they understand the methods by doing an analysis on one HF demand.

Step 5: Analyze the worst case HF demands

Now that you have a worst case HF demand condition for each HF, you use the methods and tools described in the next section to analyse those worst cases and develop the data needed to complete the appropriate column in the Human Factors and Personas Chart.

Methods and tools for specific HF demands

Specific methods and tools are available to student for assessing human factor demands and capabilities. Students are expected to use these tools and will be graded on their use.


HF demands are not explicitly reported, but they will appear in the justifications for the HF value ranges you set in the next step - establishing human factor capabilities.

We do expect details of HF demands to be included in an Appendix, insofar as such information directly supports the decisions your team makes about HFs for your design. This includes: the overall list of all HF demands found by all team members, sorted by impact; and the various calculations done on each of the worst-case HF demands per the Methods and Tools section above to show how you performed the detailed analyses.


TODO Describe consequences and counter-indications.

These are discussed in the next step of the design roadmap.
This is a Google Sheet available to students using their Ryerson accounts only.
design/human_factor_demands.txt · Last modified: 2020.10.15 13:37 by Fil Salustri