1.1.3 Percentiles

What Are Percentiles?

Percentiles are a statistical tool that helps designers understand how measurements are distributed across a population. Think of percentiles as a way to organize people from smallest to largest for any specific body measurement. In simple terms, percentiles tell us what percentage of a population falls below a certain measurement value. For example, if you're in the 70th percentile for height, it means 70% of people are shorter than you, and 30% are taller.

Why Percentiles Matter in Design

As designers, we use percentiles to:

• Determine which body dimensions to consider when creating products

• Ensure our designs accommodate an appropriate range of users

• Make informed decisions about sizing and adjustability
  

Without percentile data, we'd be designing blindly, potentially creating products that fit only a small portion of users.

Key Percentile References in Design

When working with anthropometric data, three key percentile references are particularly important:

1. 5th Percentile: Represents smaller individuals - only 5% of the population has smaller measurements. Often used for minimum dimensions (like reach distances or clearances).

2. 50th Percentile: The median or "middle" value - half the population is smaller, and half is larger. This is NOT the average but represents the typical person in a population.

3. 5th-95th Percentile Range: This range captures 90% of the population (excluding the smallest 5% and largest 5%). Designing for this range means your product will accommodate the vast majority of users.
   

Choosing the Right Percentiles for Your Design

Selecting appropriate percentiles depends on what you're designing and the critical dimensions involved:

Example 1: Classroom Chair

• Seat height: Design for the 5th percentile (smaller users) so shorter people's feet touch the ground

• Seat width: Design for the 95th percentile (larger users) so everyone fits comfortably

• Backrest height: May target the 50th percentile as a compromise if not adjustable
  

Example 2: Emergency Exit

• Door width: Must accommodate the 95th percentile (largest users)

• Handle height: Might target the 50th percentile with consideration for both shorter and taller users
  

The key is understanding which body dimensions are critical for your specific design scenario.

Demographic Factors Affecting Percentiles

It's crucial to understand that percentile data varies significantly based on demographic factors:

• Gender: Biological differences create distinct percentile distributions

• Age: Children, adults, and seniors have very different body dimensions

• Ethnicity: Body proportions vary across different populations

• Disability: Physical differences may require specialized percentile considerations
  

For example, a product designed using percentile data from adult males in Scandinavia might be completely unsuitable for children in Southeast Asia. Always consider the specific population your design targets.

Design Strategies Using Percentile Data

Once you've identified the appropriate percentiles for your design, you'll typically use one of two strategies (or a combination):

1. Adjustability: Create products that can be modified to fit different percentiles

   • Example: Car seats that adjust forward/backward, up/down, and have adjustable headrests

2. Range of Sizes: Produce multiple versions of the same product to fit different percentile groups

   • Example: Clothes in XS, S, M, L, XL sizes or shoes in different sizes
     

Practical Application in Design Projects

When working on your design projects, you'll need to:

1. Identify critical dimensions: Determine which measurements most affect usability

2. Select appropriate percentiles: Choose which percentile values to design for

3. Apply percentile data: Use the measurements to create properly sized designs

4. Test with real users: Verify your percentile choices work in practice
   

The Relationship to Other Ergonomic Concepts

Percentiles connect directly to other important design considerations:

• Work envelopes: The 3D space where users perform tasks, defined by percentile reach distances

• Clearance: Space requirements based on percentile body dimensions

• Reach: Maximum distances users can extend their limbs, varies by percentile

• Adjustability: Range needed to accommodate target percentiles

• Size variations: Different sizes needed to fit various percentile groups
  

By mastering percentiles, you'll develop the analytical skills to create designs that truly fit the humans who will use them, ensuring comfort, safety, and accessibility for your target users.